Feline bitter taste receptors and methods

ABSTRACT

A family of novel feline bitter taste receptors, referred to as feline TAS2R (fTAS2R), are disclosed herein. Isolated polynucleotides encoding the novel feline bitter taste receptors and chimeric polypeptides are also disclosed, as are expression vectors and host cells for expression of the novel feline bitter taste receptors. Methods of identifying compounds that bind to the novel feline bitter taste receptors and modulate their activity are disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. patent application Ser. No. 14/198,795, filed Mar. 6, 2014, which claims the benefit of U.S. Provisional Application No. 61/788,528, filed Mar. 15, 2013, the disclosure of each is incorporated herein by reference.

SEQUENCE LISTING

This application includes a sequence listing, incorporated herein by-reference in its entirety, submitted electronically with the application via EFS-WEB as an ASCII text file of 65 kB named 714409002 and created Feb. 18, 2014.

BACKGROUND

The taste system provides sensory information about the chemical composition of the external world. Mammals are believed to have at least five basic taste modalities: sweet, bitter, sour, salty, and umami. Each taste modality is thought to be mediated by a distinct protein receptor or receptors that are expressed in taste receptor cells found on the surface of the tongue. The taste receptors that recognize bitter, sweet, and umami taste stimuli belong to the G-protein-coupled receptor (GPCR) superfamily. Subtle differences in a receptor may alter which ligands bind and what signal is generated once the receptor is stimulated.

Various members of the GPCR superfamily mediate many other physiological functions, such as endocrine function, exocrine function, heart rate, lipolysis, and carbohydrate metabolism. The biochemical analysis and molecular cloning of a number of such receptors has revealed many basic principles regarding the domain structure and function of these receptors.

The ability of mammals to taste the five primary modalities is thought to be largely similar, however due to diet and environmental differences, taste receptors have evolved to be somewhat different across mammalian species. For example, the gene encoding the TAS1R2 protein, a component of the receptor for sweet compounds, has mutated to a nonfunctional pseudogene in felines and several other obligate carnivores, while aquatic mammals such as dolphin have lost most functional taste receptors.

The bitter taste modality is usually described as disagreeable. Many natural and synthetic toxins have been characterized as bitter tastants. As a result, it is hypothesized that bitter taste perception has evolved as a means to discourage the consumption of toxic compounds often found in plants. Estimates for the number of bitter tasting compounds are in the tens of thousands. However, only 550 compounds have been identified thus far as bitter tastants for humans. Compounds that block bitter taste perception have also been identified, for example p-(dipropylsulfamoyl)benzoic acid (probenecid) which acts on a subset of TAS2Rs.

The perception of bitter taste is mediated by TAS2R proteins, a family of monomeric G protein-coupled receptors, embedded in the surface of taste cells.

Research has shown that molecular diversity in the TAS2Rs of humans and other primates leads to functional differences in individuals' bitter taste perception (Imai et al., 2012, Biol Lett. 8(4): 652-656; Li et al., 2011, Human biology 83: 363-377). The exposure to the specific flora of a geographic region is thought to be a major driving force of selection on TAS2Rs.

Humans encode about 26 functional TAS2Rs, allowing for the detection of an enormous number of compounds. A subset of human TAS2Rs (hTAS2Rs) are currently believed to be promiscuous, i.e., activated by multiple ligands belonging to several chemical classes, while other hTAS2Rs bind ligands of only particular chemical classes. Additionally, several hTAS2Rs are orphan receptors, with no compounds identified as yet that stimulate them.

Signal transduction of bitter stimuli is accomplished via the α-subunit of gustducin. This G protein subunit activates a taste phosphodiesterase and decreases cyclic nucleotide levels. Further steps in the transduction pathway are still unknown. The βγ-subunit of gustducin also mediates taste by activating IP3 (inositol triphosphate) and DAG (diglyceride). These second messengers may open gated ion channels or may cause release of internal calcium. Though all TAS2Rs are located in gustducin-containing cells, knockout of gustducin does not completely abolish sensitivity to bitter compounds, suggesting a redundant mechanism for bitter tasting.

hTAS2R38 is the most extensively studied bitter taste receptor. Early in the twentieth century a dichotomy in the perception of phenylthiocarbamide (PTC), a bitter tasting compound, was observed in a sample of people. Most people could taste PTC, but about 25% could not. Researchers noticed the taster/non-taster phenotype had a degree of heritability. Later it was determined that the difference in phenotype between the two groups could be ascribed to a difference in genotype, more specifically single nucleotide polymorphisms (SNPs) at three positions within the hTAS2R38 DNA.

Other species display a TAS2R repertoire much different from that of humans. For example, the mouse has 34 full-length TAS2Rs encoded in its genome, while the chicken has only 3 (Go, et al. Genetics. 2005 May; 170(1):313-26). Although some compounds can be detected by multiple TAS2Rs, it is almost certain that differences in TAS2R repertoire across species result in differences in bitter taste perception.

The feline genome has been sequenced with minimal coverage. (Mullikin et al. BMC Genomics 2010 11: 406; Pontius et al., Genome Research 2007 17: 1675-1689). As a result, major gaps exist in the feline genome sequence and only slightly over 2000 feline genes have been annotated to date. As a comparison, the human genome has about 25,000 genes annotated. The sequences prior to a gap in the genomic assembly are of poor quality, so in addition to information that is missing, a large portion of the data present is of poor quality. Consequently, there is much to be discovered within feline genomics and in determining the molecular basis of feline taste perception. Not a single feline TAS2R (fTAS2R) has been annotated in the feline genome or investigated biochemically to our knowledge. Additionally, with many feline breeds originating in a particular geographic region and therefore being exposed to unique flora, breed specific TAS2R differences may exist.

There is therefore a need in the art for the identification and characterization of the feline TAS2R bitter receptors in order to understand the taste profile of felines and its modulation.

SUMMARY

Disclosed herein is an isolated polynucleotide encoding a TAS2R receptor.

In an embodiment, the isolated polynucleotide comprises the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, or SEQ ID NO:25; a nucleotide sequence encoding the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26; a nucleotide sequence substantially complementary to the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, or SEQ ID NO:25; or a nucleotide sequence that hybridizes to the complement of the polynucleotide having SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, or SEQ ID NO:25 under high stringency conditions.

An isolated TAS2R receptor polypeptide encoded by the polynucleotide, and expression vectors and host cells for expressing the encoded TAS2R receptor are also disclosed.

In an embodiment, the isolated TAS2R receptor polypeptide comprises at least one extracellular domain of a feline TAS2R receptor; at least one transmembrane domain of a feline TAS2R receptor; or at least one intracellular domain of a feline TAS2R receptor, wherein the feline TAS2R receptor comprises the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26.

In an embodiment, the isolated TAS2R receptor polypeptide comprises the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26.

Also disclosed herein is a chimeric polypeptide comprising an extracellular domain or a transmembrane region of a feline TAS2R receptor polypeptide linked to a heterologous polypeptide.

In an embodiment, the chimeric polypeptide comprises an extracellular domain of a feline TAS2R receptor polypeptide comprising amino acids 1, 68-84; 146-179; or 249-257 of SEQ ID NO:2; amino acids 1-10, 73-88; 151-186; or 256-264 of SEQ ID NO:4; amino acids 1-8; 72-88; 150-186; or 256-265 of SEQ ID NO:6; amino acids 1-2; 69-87; 151-183; or 253-261 of SEQ ID NO:8; amino acids 1-8; 72-88; 150-187; or 257-265 of SEQ ID NO:10; amino acids 1-6; 72-88; 150-183; or 253-262 of SEQ ID NO:12; amino acids 1; 69-87; 150-181; or 251-260 of SEQ ID NO:14; amino acids 1-8; 69-88; 150-185; or 252-261 of SEQ ID NO:16; amino acids 1-17: 83-98; 161-198; or 268-277 of SEQ ID NO:18; amino acids 1; 69-88; 150-185; or 255-264 of SEQ ID NO:20; amino acids 1-2; 69-87; 149-181; or 251-260 of SEQ ID NO:22; amino acids 1-2; 69-87; 149-181; or 251-259 of SEQ ID NO:24; or amino acids 1-8; 72-88; 150-185; or 254-263 of SEQ ID NO:26; or a transmembrane region of the feline TAS2R receptor polypeptide comprising amino acids 2-22, 47-67, 85-105, 125-145, 180-200, 228-248, or 258-278 of SEQ ID NO:2; amino acids 11-31, 52-72, 89-109, 130-150, 187-207, 235-255, or 265-285 of SEQ ID NO:4; amino acids 9-29, 51-71, 89-109, 129-149, 187-207, 235-255, or 266-286 of SEQ ID NO:6; amino acids 3-23, 48-68, 88-108, 130-150, 184-204, 232-252, or 262-282 of SEQ ID NO:8; amino acids 9-29, 51-71, 89-109, 129-149, 188-208, 236-256, or 266-286 of SEQ ID NO:10; amino acids 7-27, 51-71, 89-109, 129-149, 184-204, 232-252, or 263-283 of SEQ ID NO:12; amino acids 2-22, 48-68, 88-108, 129-149, 182-202, 230-250, or 261-281 of SEQ ID NO:14; amino acids 9-29, 48-68, 89-109, 129-149, 186-206, 231-251, or 262-282 of SEQ ID NO:16; amino acids 18-38, 62-82, 99-119, 140-160, 199-219, 247-267, or 278-298 of SEQ ID NO:18; amino acids 2-22, 48-68, 89-109, 129-149, 186-206, 234-254, or 265-285 of SEQ ID NO:20; amino acids 3-23, 48-68, 88-108, 128-148, 182-202, 230-250, or 261-281 of SEQ ID NO:22; amino acids 3-23, 48-68, 88-108, 128-148, 182-202, 230-250, or 260-280 of SEQ ID NO:24; or amino acids 9-29, 51-71, 89-109, 129-149, 186-206, 233-253, or 264-284 of SEQ ID NO:26; linked to a heterologous polypeptide.

Antibodies and kits for detecting the fTAS2R receptor or the polynucleotide encoding the fTAS2R receptor are also disclosed.

Also disclosed herein are methods for identifying compounds that interact with a fTAS2R receptor.

In an embodiment, the method comprises contacting a fTAS2R receptor herein with a test compound, and detecting interaction between the receptor and the compound.

In an embodiment, the method comprises contacting a fTAS2R receptor with a receptor ligand in the presence or absence of a test compound, and determining whether the test compound modulates binding of the ligand to the receptor or activation of the receptor by the ligand.

In an embodiment, the method comprises contacting a fTAS2R receptor herein with a test compound, and detecting an increase in biological activity of the receptor in the presence of the compound relative to biological activity of the polypeptide in the absence of the compound.

In an embodiment, the method comprises contacting a fTAS2R receptor herein with a test compound, and detecting a decrease in biological activity of the receptor in the presence of the compound relative to biological activity of the polypeptide in the absence of the compound.

These and other advantages, as well as additional inventive features, will be apparent from the following Drawings, Detailed Description, Examples, and Claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sequence alignment displaying the 3rd through the 7th transmembrane (TM) region (transmembrane regions in grey) of several human and feline bitter receptors: human TAS2R16 (SEQ ID NO:30), TAS2R4 (SEQ ID NO:27), TAS2R9 (SEQ ID NO:28), TAS2R10 (SEQ ID NO:29) AND TAS2R38 (SEQ ID NO:31); and feline bitter receptors, TAS2R4 (SEQ ID NO:8), 9 (SEQ ID NO:12), 10 (SEQ ID NO:14), 12 (SEQ ID NO:16), and 38 (SEQ ID NO:18).

FIG. 2 shows a sequence alignment for human TAS2R38 polypeptide (SEQ ID NO:31) and feline TAS2R38 polypeptide (SEQ ID NO:18) determined from sequencing of genomic DNA of five individual cats.

DETAILED DESCRIPTION

A family of novel feline bitter taste receptors, referred to as feline TAS2R (fTAS2R), are disclosed herein. These G-protein coupled receptors (GPCRs) are components of the feline taste transduction pathway, specifically, part of the bitter taste transduction pathway, and are involved in feline taste detection of bitter substances such as 6-n-propylthiouracil, sucrose octaacetate, raffinose undecaacetate, cycloheximide, denatonium, copper glycinate, and quinine Isolated polynucleotides encoding the novel feline bitter taste receptors are also disclosed, as are expression vectors and host cells for expression of the novel feline bitter taste receptors. Methods of expressing and isolating the nucleic acids and encoded polypeptides are also disclosed.

The nucleic acids provide probes for identification of cells in which the nucleic acids are expressed, e.g., taste cells. For example, probes for expression of TAS2R polypeptides can be used to identity taste cells present in foliate, circumvallate, and fungiform papillae. In particular, the TAS2R probes are useful to identify bitter sensing cells and can serve as tools for the generation of anatomical maps that elucidate the relationship between the bitter sensing cells and their projections into the central nervous system. Methods of identifying compounds that bind to the novel feline bitter taste receptors and modulate their activity are disclosed. In the methods, members of the fTAS2R family act as direct or indirect reporter molecules to identify modulators of taste receptor expressing cellular activity. Such compounds that modulate feline bitter taste receptor activity are useful for modulation of feline bitter taste receptor activity. Modulators can be activators, inhibitors, enhancers, agonists, and/or antagonists of the feline bitter receptors. These modulatory compounds can be used in the food and pharmaceutical industries to customize taste of foods or drugs, for example, to decrease the bitter taste of foods or drugs. Thus, the methods disclosed herein are useful for designing formulations of feline food, feline food palatants, feline treats, and feline medications in which aversive compounds are avoided or blocked.

Bitter taste perception is mediated by G protein-coupled receptors (GPCRs) of the taste receptor 2 family (TAS2R). The TAS2R genes encode a family of related seven transmembrane G-protein coupled receptors involved in taste transduction, which interact with a G-protein to mediate taste signal transduction. In particular, TAS2Rs interact in a ligand-specific manner with the G protein Gustducin.

To date, much work has been done to characterize human TAS2Rs (hTAS2Rs). The human genome encodes about 26 functional TAS2Rs that are glycoproteins. All hTAS2Rs share a conserved site for Asn-linked glycosylation within the center of the second extracellular loop. The hTAS2Rs also have the ability to form homo- and hetero-oligomers with other GPCR when expressed in vitro, however at present no evidence exists that TAS2R receptor oligomerization has functional implications.

Bitter taste receptor cells represent a distinct subpopulation of chemosensory cells characterized by the expression of TAS2R genes and completely segregated from those receptor cells devoted to the detection of other taste stimuli. Each bitter taste receptor cell expresses multiple bitter taste receptors, although the extent of co-expression is still a matter of debate.

In addition to their expression in the gustatory system, TAS2Rs are found in non-gustatory tissues. Among these extraoral sites are the respiratory epithelia, gastrointestinal tissues, reproductive organs, and brain. Bitter taste receptors are implicated in differentiation or maturation of sperm in mice. The non-gustatory expression of TAS2Rs is known to be used to regulate digestion and respiration.

Activation of TAS2R receptors in an enteroendocrine cell line (STC-1 cells) results in release of the peptide hormone cholecystokinin (CCK), which can reduce gut motility. Consequently, intake of a potential toxin that activates the TAS2R pathway may decrease the rate at which food passes through the stomach and lower the drive for continued eating. The release of CCK also excites sensory nerve processes of the vagus nerve to carry the signal to the brain, suggesting that regulation of food intake involves both peripheral and central controls. Activation of the TAS2R signaling network may also or alternatively indirectly increase elimination of absorbed toxins from gut epithelium before the toxins can enter circulation since some data suggest that the CCK-secreting enteroendocrine cells are involved in a paracrine signaling system that reduces transfer of toxic substances from the gut into the circulation. Lower in the gut, activation of TAS2R receptors has a different effect. When some bitter-tasting ligands are applied to the colonic epithelium, they induce the secretion of anions, which leads to fluid secretion by the epithelium which may flush out any noxious irritant from the colon.

Solitary chemosensory cells (SCCs) are also present throughout the upper respiratory system and express the entire suite of taste-related signaling molecules, including TAS2R receptors, PLCβ2, gustducin, and the transduction channel TrpM5. The SCCs synapse onto polymodal pain fibers of the trigeminal nerve. Inhalation of a toxin that activates TAS2R receptors of the SCCs will be irritating and evoke trigeminally-mediated reflex changes in respiration. Additionally, the activated trigeminal nerve fibers release peptide modulators that result in local neurogenic inflammation of the respiratory epithelium, activating the immune system in response to the presence of the toxins.

Ligands of a mammalian TAS2R1 can include adhumulone, adlupulone, amarogentin, arborescin, cascarillin, chloramphenicol, cis-isocohumulone, cis-isoloadhumulone, cohumulone, colupulone, dextromethorphan, diphenidol (diphenylthiourea, sulfocarbanilide, sym-diphenylthiourea, or thiocarbanilide), humulon (humulone), isoxanthohumol, lupulon, lupulone, parthenolide, picrotoxinin, sodium cyclamate, sodium thiocyanate, thiamine, trans-isoadhumulone, trans-isocohumulone, trans-isohumulone, xanthohumol, and yohimbine. The mammalian TAS2R1 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R1 is a feline TAS2R1.

Ligands of a mammalian TAS2R3 can include chloroquine. The mammalian TAS2R3 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R3 is a feline TAS2R3.

Ligands of a mammalian TAS2R4 can include amarogentin, arborescin, artemorin, azathioprine, brucine, campher, chlorpheniramine, colchicine, dapsone, denatonium benzoate, diphenidol, parthenolide, quassin, quinine, and yohimbine. The mammalian TAS2R4 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R4 is a feline TAS2R4.

Ligands of a mammalian TAS2R7 can include caffeine, chlorpheniramine, cromolyn, diphenidol, papaverine, and quinine. The mammalian TAS2R7 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R7 is a feline TAS2R7.

Ligands of a mammalian TAS2R9 can include ofloxacin, pirenzapin, and procainamid. The mammalian TAS2R9 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R9 is a feline TAS2R9.

Ligands of a mammalian TAS2R10 can include (−)-alpha thujone, absinthin, arborescin, arglabin, artemorin, azathioprine, benzoin, caffeine, campher, cascarillin, chloramphenicol, chloroquine, chlorpheniramine, coumarin, cucurbitacin b, cucurbitacin e, cucurbitacins, cycloheximid, cycloheximide, dapsone, denatonium benzoate, dextromethorphan, diphenidol, erythromycin, famotidine, haloperidol, papaverine, parthenolide, picrotoxinin, quassin, quinine, strychnine, and yohimbine. The mammalian TAS2R10 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R10 is a feline TAS2R10.

Ligands of a mammalian TAS2R38 can include 6-methyl-2-thiouracil, acetylthiourea, allyl isothiocyanate, caprolactam, chlorpheniramine, dimethylthioformamide, diphenidol, (diphenylthiourea, sulfocarbanilide, sym-diphenylthiourea, thiocarbanilide), ethylene thiourea, n,n-ethylene thiourea, ethylpyrazine, limonin, methimazole, n-ethylthiourea, n-methylthiourea, phenethyl isothiocyanate, phenylthiocarbamide (ptc), propylthiouracil, sinigrin, sodium cyclamate, sodium_thiocyanate, and yohimbine. The mammalian TAS2R38 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R38 is a feline TAS2R38.

Ligands of a mammalian TAS2R43 can include acesulfame K, aloin, amarogentin, arborescin, arglabin, aristolochic acid, caffeine, chloramphenicol, cromolyn, denatonium benzoate, diphenidol, falcarindiol, grosheimin (grossheimin), helicin, quinine, and saccharin. The mammalian TAS2R43 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R43 is a feline TAS2R43.

Ligands of a mammalian TAS2R44 can include acesulfame K, aloin, aristolochic acid, diphenidol, famotidine, parthenolide, quinine, and saccharin. The mammalian TAS2R44 can be from a human, a rodent, a canine, or a feline. In an embodiment, the mammalian TAS2R44 is a feline TAS2R44.

The human bitter taste receptors, hTAS2R2, hTAS2R41, hTAS2R42, hTAS2R45, hTAS2R48, and hTAS2R60 are still considered orphan GPCRs since ligands have not yet been identified for these receptors.

Until recently, hTAS2R2 was annotated as a pseudogene due to a two base deletion at codon 160 found in sequences collected from 10 human populations (Karitiana, Surui, Waorani Indians from South America, Russians from Eastern Europe, Druze from the Middle East, Atayal, Chinese, Japanese from Eastern Asia, and Khmers and Melanesians from Southeast Asia) and from GenBank resources. hTAS2R2 has been found to be polymorphic with respect to that deletion, with the intact gene found in the Adygei (Eastern European), Mbuti (African Pygmies), and Biaka (African Pygmies) (Go Y et al., Genetics May 1, 2005, 170 (1): 313-326).

Human TAS2R (hTAS2R) gene and pseudogene nucleotide sequences were used as references to identify, via a bioinformatics approach, previously unknown feline TAS2R (fTAS2R) genes. Subsequently, isolated feline genomic DNA was used to clone the fTAS2R genes. The nucleotide sequence of the cloned fTAS2R genes of several felines was then determined by sequencing, e.g., Sanger sequencing, and used to establish a consensus nucleotide sequence for the gene, and to identify any variant sites in the sequence.

An isolated polynucleotide encoding a fTAS2R receptor is disclosed. The isolated polynucleotide can comprise the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, or SEQ ID NO:25; a nucleotide sequence encoding the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26; a nucleotide sequence substantially complementary to the nucleotide sequence of SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, or SEQ ID NO:25; or a nucleotide sequence that hybridizes to the complement of the polynucleotide having SEQ ID NO:1, SEQ ID NO:3, SEQ ID NO:5, SEQ ID NO:7, SEQ ID NO:9, SEQ ID NO:11, SEQ ID NO:13, SEQ ID NO:15, SEQ ID NO:17, SEQ ID NO:19, SEQ ID NO:21, SEQ ID NO:23, or SEQ ID NO:25 under high stringency conditions.

Further disclosed is a polynucleotide comprising a sequence having at least 90%, at least 95%, at least 97%, at least 98%, at least 99% homology with SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, or the complement of SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, or 25.

A single nucleotide polymorphism in the nucleic acid sequence encoding fTAS2R38 was identified at nucleotide 220 from sequencing amplified feline genomic DNA from multiple subjects. The two alleles observed at the nucleotide 220 were G and A. The G220A nucleic acid variation corresponds to an amino acid variation D74N in the fTAS2R38 protein sequence.

In another aspect, an isolated fTAS2R receptor polypeptide is disclosed.

In an embodiment, the isolated fTAS2R polypeptide is encoded by a polynucleotide disclosed herein.

In an embodiment, the isolated fTAS2R polypeptide can comprise the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26; or an amino acid sequence having at least 90%, at least 95%, at least 97%, at least 98%, at least 99% homology with the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26.

Sensory GPCRs, such as the TAS2R bitter taste receptors, have a domain structure including an “N-terminal domain”; “extracellular domains”; a “transmembrane domain” comprising seven transmembrane regions, cytoplasmic, and extracellular loops; “cytoplasmic domains”; and a “C-terminal domain”. These domains can be structurally identified using methods known in the art, such as sequence analysis programs that identify hydrophobic and hydrophilic domains. Such domains are useful for making chimeric proteins and for in vitro assays disclosed herein, e.g., ligand binding assays.

The term “extracellular domains” refers to the domains of TAS2R polypeptides that protrude from the cellular membrane and are exposed to the extracellular face of the cell. Such domains would include the “N terminal domain” that is exposed to the extracellular face of the cell, as well as the extracellular loops of the transmembrane domain that are exposed to the extracellular face of the cell, i. e., the loops between transmembrane regions 2 and 3, and between transmembrane regions 4 and 5.

The “N terminal domain” region starts at the N-terminus and extends to a region close to the start of the transmembrane domain. These extracellular domains are useful for in vitro ligand binding assays, both soluble and solid phase.

The “transmembrane domain,” which comprises the seven transmembrane regions, refers to the domain of TAS2R polypeptides that lies within the plasma membrane, and may also include the corresponding cytoplasmic (intracellular) and extracellular loops, also referred to as transmembrane domain “regions.” Transmembrane regions can also bind ligand either in combination with the extracellular domain or alone, and are therefore also useful for in vitro ligand binding assays.

The seven transmembrane regions and extracellular and cytoplasmic loops can be identified using standard methods known in the art. For example, transmembrane regions of the fTAS2R proteins can be identified using software, TOPCONS, available on the internet from the Stockholm Bioinformatics Center, Stockholm University (Andreas Bernsel, et al. (2009) Nucleic Acids Research 37 (Webserver issue), W465-8). The seven transmembrane regions and extracellular and cytoplasmic loops of the fTAS2R identified by TOPCONS are shown in the following table:

TABLE 1 TOPCONS prediction of seven transmembrane regions, extracellular loops, and intracellular loops fTAS2R1 (SEQ ID NO: 2) Transmembrane domains: 1: 2-22, 2: 47-67, 3: 85-105, 4: 125-145, 5: 180-200, 6: 228-248, 7: 258-278 Extracellular domain: 1, 68-84; 146-179; 249-257 Intracellular domain: 23-46; 106-124; 201-227; 279-298 fTAS2R2 (SEQ ID NO: 4) Transmembrane domains: 1: 11-31, 2: 52-72, 3: 89-109, 4: 130-150, 5: 187-207, 6: 235-255, 7: 265-285 Extracellular domain: 1-10, 73-88; 151-186; 256-264 Intracellular domain: 32-51; 110-129; 208-234; 286-304 fTAS2R3 (SEQ ID NO: 6) Transmembrane domains: 1: 9-29, 2: 51-71, 3: 89-109, 4: 129-149, 5: 187-207, 6: 235-255, 7: 266-286 Extracellular domain: 1-8; 72-88; 150-186; 256-265 Intracellular domain: 30-50; 110-128; 208-234; 287-316 fTAS2R4 (SEQ ID NO: 8) Transmembrane domains: 1: 3-23, 2: 48-68, 3: 88-108, 4: 130-150, 5: 184-204, 6: 232-252, 7: 262-282 Extracellular domain: 1-2; 69-87; 151-183; 253-261 Intracellular domain: 24-47; 109-129; 205-231; 283-306 fTAS2R7 (SEQ ID NO: 10) Transmembrane domains: 1: 9-29, 2: 51-71, 3: 89-109, 4: 129-149, 5: 188-208, 6: 236-256, 7: 266-286 Extracellular domain: 1-8; 72-88; 150-187; 257-265 Intracellular domain: 30-50; 110-128; 209-235; 287-311 fTAS2R9 (SEQ ID NO: 12) Transmembrane domains: 1: 7-27, 2: 51-71, 3: 89-109, 4: 129-149, 5: 184-204, 6: 232-252, 7: 263-283 Extracellular domain: 1-6; 72-88; 150-183; 253-262 Intracellular domain: 28-50; 110-128; 205-231; 284-337 fTAS2R10 (SEQ ID NO: 14) Transmembrane domains: 1: 2-22, 2: 48-68, 3: 88-108, 4: 129-149, 5: 182-202, 6: 230-250, 7: 261-281 Extracellular domain: 1; 69-87; 150-181; 251-260 Intracellular domain: 23-48; 109-128; 203-229; 282-300 fTAS2R12 (SEQ ID NO: 16) Transmembrane domains: 1: 9-29, 2: 48-68, 3: 89-109, 4: 129-149, 5: 186-206, 6: 231-251, 7: 262-282 Extracellular domain: 1-8; 69-88; 150-185; 252-261 Intracellular domain: 30-47; 110-128; 207-230; 283-309 fTAS2R38 (SEQ ID NO: 18) Transmembrane domains: 1: 18-38, 2: 62-82, 3: 99-119, 4: 140-160, 5: 199-219, 6: 247-267, 7: 278-298 Extracellular domain: 1-17: 83-98; 161-198; 268-277 Intracellular domain: 39-61; 120-139; 220-246; 299-334 fTAS2R42 (SEQ ID NO: 20) Transmembrane domains: 1: 2-22, 2: 48-68, 3: 89-109, 4: 129-149, 5: 186-206, 6: 234-254, 7: 265-285 Extracellular domain: 1; 69-88; 150-185; 255-264 Intracellular domain: 23-47; 110-128; 207-233; 286-322 fTAS2R43 (SEQ ID NO: 22) Transmembrane domains: 1: 3-23, 2: 48-68, 3: 88-108, 4: 128-148, 5: 182-202, 6: 230-250, 7: 261-281 Extracellular domain: 1-2; 69-87; 149-181; 251-260 Intracellular domain: 24-47; 109-127; 203-229; 282-299 fTAS2R44 (SEQ ID NO: 24) Transmembrane domains: 1: 3-23, 2: 48-68, 3: 88-108, 4: 128-148, 5: 182-202, 6: 230-250, 7: 260-280 Extracellular domain: 1-2; 69-87; 149-181; 251-259 Intracellular domain: 24-47; 109-127; 203-229; 281-308 fTAS2R67 (SEQ ID NO: 26) Transmembrane domains: 1: 9-29, 2: 51-71, 3: 89-109, 4: 129-149, 5: 186-206, 6: 233-253, 7: 264-284 Extracellular domain: 1-8; 72-88; 150-185; 254-263 Intracellular domain: 30-50; 110-128; 207-232; 285-312

Alternative predictions of the transmembrane regions and extracellular and cytoplasmic loops of the fTAS2R proteins can be generated using different software also available on the internet from the Stockholm Bioinformatics Center, including SCAMPI (Andreas Bernsel, et al. (2008) Proc. Natl. Acad. Sci. USA. 105, 7177-7181.); PRODIV (Håkan Viklund and Arne Elofsson (2004) Protein Science 13, 1908-1917), and OCTAPUS (Håkan Viklund and Arne Elofsson (2008) Bioinformatics. 24, 1662-1668.) Additional methods known in the art to predict the structural regions include hydropathy prediction methods of Goldman-Engleman-Steitz, or Kyte-Doolittle (J. Mol. Biol. 157: 105-132 (1982), or Hopp-Woods. Secondary structure prediction methods include Garnier-Robson, or Deléage & Roux or Chou-Fasman. As known in the art, the various available algorithms may predict slightly different boundaries for transmembrane regions based on the amino acid sequence.

The term “transmembrane region” as used herein denotes a three-dimensional protein structure which is thermodynamically stable in a membrane, e.g., a single transmembrane alpha helix or a transmembrane beta barrel.

“Cytoplasmic domains” refers to the domains of TAS2R proteins that face the inside of the cell, e. g., the “C terminal domain” and the intracellular loops of the transmembrane domain, e. g., the intracellular loops between transmembrane regions 1 and 2, the intracellular loops between transmembrane regions 3 and 4, and the intracellular loops between transmembrane regions 5 and 6.

“C terminal domain” refers to the region that spans the end of the last transmembrane domain and the C-terminus of the protein, and which is normally located within the cytoplasm.

In an embodiment, the isolated TAS2R receptor polypeptide can comprise at least one extracellular domain of a feline TAS2R receptor; at least one transmembrane domain of a feline TAS2R receptor; or at least one intracellular domain of a feline TAS2R receptor, wherein the feline TAS2R receptor comprises the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26; or an amino acid sequence having at least 90% homology, specifically at least 95% homology, more specifically at least 97% homology with the amino acid sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26.

The extracellular domain of the fTAS2R polypeptide can comprise amino acids 1, 68-84; 146-179; or 249-257 of SEQ ID NO:2; amino acids 1-10, 73-88; 151-186; or 256-264 of SEQ ID NO:4; amino acids 1-8; 72-88; 150-186; or 256-265 of SEQ ID NO:6; amino acids 1-2; 69-87; 151-183; or 253-261 of SEQ ID NO:8; amino acids 1-8; 72-88; 150-187; or 257-265 of SEQ ID NO:10; amino acids 1-6; 72-88; 150-183; or 253-262 of SEQ ID NO:12; amino acids 1; 69-87; 150-181; or 251-260 of SEQ ID NO:14; amino acids 1-8; 69-88; 150-185; or 252-261 of SEQ ID NO:16; amino acids 1-17: 83-98; 161-198; or 268-277 of SEQ ID NO:18; amino acids 1; 69-88; 150-185; or 255-264 of SEQ ID NO:20; amino acids 1-2; 69-87; 149-181; or 251-260 of SEQ ID NO:22; amino acids 1-2; 69-87; 149-181; or 251-259 of SEQ ID NO:24; or amino acids 1-8; 72-88; 150-185; or 254-263 of SEQ ID NO:26.

The transmembrane domain of the fTAS2R polypeptide can comprise amino acids 2-22, 47-67, 85-105, 125-145, 180-200, 228-248, or 258-278 of SEQ ID NO:2; amino acids 11-31, 52-72, 89-109, 130-150, 187-207, 235-255, or 265-285 of SEQ ID NO:4; amino acids 9-29, 51-71, 89-109, 129-149, 187-207, 235-255, or 266-286 of SEQ ID NO:6; amino acids 3-23, 48-68, 88-108, 130-150, 184-204, 232-252, or 262-282 of SEQ ID NO:8; amino acids 9-29, 51-71, 89-109, 129-149, 188-208, 236-256, or 266-286 of SEQ ID NO:10; amino acids 7-27, 51-71, 89-109, 129-149, 184-204, 232-252, or 263-283 of SEQ ID NO:12; amino acids 2-22, 48-68, 88-108, 129-149, 182-202, 230-250, or 261-281 of SEQ ID NO:14; amino acids 9-29, 48-68, 89-109, 129-149, 186-206, 231-251, or 262-282 of SEQ ID NO:16; amino acids 18-38, 62-82, 99-119, 140-160, 199-219, 247-267, or 278-298 of SEQ ID NO:18; amino acids 2-22, 48-68, 89-109, 129-149, 186-206, 234-254, or 265-285 of SEQ ID NO:20; amino acids 3-23, 48-68, 88-108, 128-148, 182-202, 230-250, or 261-281 of SEQ ID NO:22; amino acids 3-23, 48-68, 88-108, 128-148, 182-202, 230-250, or 260-280 of SEQ ID NO:24; or amino acids 9-29, 51-71, 89-109, 129-149, 186-206, 233-253, or 264-284 of SEQ ID NO:26.

The intracellular domain of the fTAS2R polypeptide can comprise amino acids 23-46; 106-124; 201-227; or 279-298 of SEQ ID NO:2; amino acids 32-51; 110-129; 208-234; or 286-304 of SEQ ID NO:4; amino acids 30-50; 110-128; 208-234; or 287-316 of SEQ ID NO:6; amino acids 24-47; 109-129; 205-231; or 283-306 of SEQ ID NO:8; amino acids 30-50; 110-128; 209-235; or 287-311 of SEQ ID NO:10; amino acids 28-50; 110-128; 205-231; or 284-337 of SEQ ID NO:12; amino acids 23-48; 109-128; 203-229; or 282-300 of SEQ ID NO:14; amino acids 30-47; 110-128; 207-230; or 283-309 of SEQ ID NO:16; amino acids 39-61; 120-139; 220-246; or 299-334 of SEQ ID NO:18; amino acids 23-47; 110-128; 207-233; or 286-322 of SEQ ID NO:20; amino acids 24-47; 109-127; 203-229; or 282-299 of SEQ ID NO:22; amino acids 24-47; 109-127; 203-229; or 281-308 of SEQ ID NO:24; or amino acids 30-50; 110-128; 207-232; or 285-312 of SEQ ID NO:26.

Also disclosed is a polynucleotide encoding the polypeptide comprising at least one extracellular domain of a feline TAS2R receptor; at least one transmembrane domain of a feline TAS2R receptor; or at least one intracellular domain of a feline TAS2R receptor.

In another aspect, a chimeric polypeptide comprising an extracellular domain or a transmembrane region of a feline TAS2R receptor polypeptide linked to a heterologous polypeptide is disclosed. The extracellular domain or the transmembrane region of the feline TAS2R receptor polypeptide can be any of those disclosed herein.

The heterologous polypeptide can be any suitable polypeptide known in the art. The heterologous polypeptide can be, for example, a sequence to determine cellular localization and expression, to permit proper folding of the chimeric polypeptide in an expression system, and/or to facilitate isolation of the chimeric polypeptide. The heterologous polypeptide can be linked to the amino terminal end or the carboxy terminal end of the fTAS2R sequence. For example, the heterologous polypeptide can be the first 45 amino acids of rat somatostatin, the FLAG® tag, a 6× histidine (his) tag, MYC, a fluorescent protein tag, V5, and/or glutathione S-transferase (GST). When the heterologous polypeptide is the first 45 amino acids of rat somatostatin, it is typically placed at the amino terminal end of the chimeric polypeptide to permit membrane targeting. When the heterologous polypeptide is a tag to permit easier isolation of the chimeric polypeptide, e.g., a 6× histidine tag, it can be placed at the amino terminus of the chimeric polypeptide. Determination of a suitable location for the heterologous polypeptide in the chimeric polypeptide relative to the amino end or the carboxy end of the fTAS2R sequence to obtain a particular functional aspect of the heterologous polypeptide on the chimeric polypeptide can be made by one of skill in the art.

Also disclosed is a polynucleotide encoding the chimeric polypeptide.

The terms “polypeptide,” “peptide”, and “protein” are used interchangeably herein to refer to a molecule formed from the linking, in a defined order, of at least two amino acids. The link between one amino acid residue and the next is an amide bond and is sometimes referred to as a peptide bond. A polypeptide can be obtained by a suitable method known in the art, including isolation from natural sources, expression in a recombinant expression system, chemical synthesis, or enzymatic synthesis. The terms also apply to amino acid polymers in which one or more amino acid residue is an artificial chemical mimetic of a corresponding naturally occurring amino acid, as well as to naturally occurring amino acid polymers and non-naturally occurring amino acid polymer.

The term “amino acid” refers to naturally occurring and synthetic amino acids, as well as amino acid analogs and amino acid mimetics that function in a manner similar to the naturally occurring amino acids. Naturally occurring amino acids are those encoded by the genetic code, as well as those amino acids that are later modified, e.g., hydroxyproline, gamma-carboxyglutamate, and O-phosphoserine. Amino acid analogs refers to compounds that have the same basic chemical structure as a naturally occurring amino acid, i.e., a carbon that is bound to a hydrogen, a carboxyl group, an amino group, and an R group, e.g., homoserine, norleucine, methionine sulfoxide, methionine methyl sulfonium. Such analogs have modified R groups (e.g., norleucine) or modified peptide backbones, but retain the same basic chemical structure as a naturally occurring amino acid. Amino acid mimetics means chemical compounds that have a structure that is different from the general chemical structure of an amino acid, but that functions in a manner similar to a naturally occurring amino acid.

Amino acids may be referred to herein by either their commonly known three letter symbols or by the one-letter symbols recommended by the IUPAC-IUB Biochemical Nomenclature Commission.

Unless otherwise indicated, a particular polypeptide sequence also implicitly encompasses conservatively modified variants thereof. A conservative amino acid substitution in a polypeptide sequence includes the substitution of an amino acid in one class by an amino acid of the same class, where a class is defined by common physicochemical amino acid side chain properties and high substitution frequencies in homologous proteins found in nature, as determined, for example, by a standard Dayhoff frequency exchange matrix or BLOSUM matrix. Six general classes of amino acid side chains have been categorized and include: Class I (Cys); Class II (Ser, Thr, Pro, Ala, Gly); Class III (Asn, Asp, Gln, Glu); Class IV (His, Arg, Lys); Class V (Ile, Leu, Val, Met); and Class VI (Phe, Tyr, Trp). For example, substitution of an Asp for another class III residue such as Asn, Gln, or Glu, is a conservative substitution. One of skill in the art can readily determine regions of the molecule of interest that can tolerate change by reference to Hopp/Woods and Kyte-Doolittle plots.

Macromolecular structures such as polypeptide structures can be described in terms of various levels of organization. “Primary structure” refers to the amino acid sequence of a particular peptide. “Secondary structure” refers to locally ordered, three dimensional structures within a polypeptide. These structures are commonly known as domains. Domains are portions of a polypeptide that form a compact unit of the polypeptide and are typically 50 to 350 amino acids long. Typical domains are made up of sections of lesser organization such as stretches of .beta.-sheet and .alpha.-helices. “Tertiary structure” refers to the complete three dimensional structure of a polypeptide monomer. “Quaternary structure” refers to the three dimensional structure formed by the noncovalent association of independent tertiary units.

The term “chimeric polypeptide” refers to a molecule, which does not occur in isolated form in nature, in which all or a portion of an fTAS2R polypeptide sequence is part of the linear chimeric polypeptide sequence. The portion of an fTAS2R polypeptide sequence can be the amino acid sequence of one or more domains of the complete fTAS2R polypeptide. For example, the portion can be an extracellular domain of a fTAS2R polypeptide. The chimeric polypeptide can be made by any method known in the art. For example, the chimeric polypeptide can be made by a recombinant expression system or can be synthesized.

The terms “isolated” or “purified”, used interchangeably herein, refers to a nucleic acid, a polypeptide, or other biological moiety that is removed from components with which it is naturally associated. The term “isolated” can refer to a polypeptide that is separate and discrete from the whole organism with which the molecule is found in nature or is present in the substantial absence of other biological macro-molecules of the same type. The term “isolated” with respect to a polynucleotide can refer to a nucleic acid molecule devoid, in whole or part, of sequences normally associated with it in nature; or a sequence, as it exists in nature, but having heterologous sequences in association therewith; or a molecule disassociated from the chromosome. Purity and homogeneity are typically determined using analytical chemistry techniques, for example polyacrylamide gel electrophoresis or high performance liquid chromatography. A protein that is the predominant species present in a preparation is substantially purified. In particular, an isolated TAS2R nucleic acid is separated from open reading frames that flank the TAS2R gene and encode proteins other than a TAS2R. In some embodiments, the term “purified” means that the nucleic acid or protein is at least 85% pure, specifically at least 90% pure, more specifically at least 95% pure, or yet more specifically at least 99% pure.

The term “nucleic acid” or “polynucleotide” includes DNA molecules and RNA molecules. A polynucleotide may be single-stranded or double-stranded. Polynucleotides can contain known nucleotide analogs or modified backbone residues or linkages, which are synthetic, naturally occurring, and non-naturally occurring, which have similar binding properties as the reference nucleic acid. Examples of such analogs include, without limitation, phosphorothioates, phosphoramidates, methyl phosphonates, chiral-methyl phosphonates, 2-O-methyl ribonucleotides, peptide-nucleic acids (PNAs). A polynucleotide can be obtained by a suitable method known in the art, including isolation from natural sources, chemical synthesis, or enzymatic synthesis. Nucleotides may be referred to by their commonly accepted single-letter codes.

Unless otherwise indicated, a particular nucleic acid sequence also implicitly encompasses conservatively modified variants thereof (e.g. degenerate codon substitutions) and complementary sequences, as well as the sequence explicitly indicated. Specifically, degenerate codon substitutions may be achieved by generating sequences in which the third position of one or more selected (or all) codons is substituted with mixed-base and/or deoxyinosine residues (Batzer et al., Nucleic Acid Res. 19:5081 (1991); Ohtsuka et al., J. Biol. Chem. 260:2605-2608 (1985); Rossolini et al., Mol. Cell. Probes 8:91-98 (1994)).

“Homology” refers to the percent identity between two polynucleotide or two polypeptide moieties. Two DNA, or two polypeptide sequences are “substantially homologous” to each other when the sequences exhibit at least about 50%, specifically at least about 75%, more specifically at least about 80%-85%, at least about 90%, and most specifically at least about 95%-98% sequence identity over a defined length of the molecules. As used herein, substantially homologous also refers to sequences showing complete identity to the specified DNA or polypeptide sequence.

In general, “identity” refers to an exact nucleotide-to-nucleotide or amino acid-to-amino acid correspondence of two polynucleotides or polypeptide sequences, respectively. Percent identity can be determined by a direct comparison of the sequence information between two molecules by aligning the sequences, counting the exact number of matches between the two aligned sequences, dividing by the length of the shorter sequence, and multiplying the result by 100. Readily available computer programs can be used to aid in the analysis, such as ALIGN (Dayhoff, M. O. in Atlas of Protein Sequence and Structure M. O. Dayhoff ed., 5 Suppl. 3:353-358, National Biomedical Research Foundation, Washington, D.C.), which adapts the local homology algorithm of Smith and Waterman 1981 Advances in Appl Math 2:482-489, for peptide analysis. Programs for determining nucleotide sequence identity are available in the Wisconsin Sequence Analysis Package, Version 8 (available from Genetics Computer Group, Madison, Wis.) for example, the BESTFIT, FASTA and GAP programs, which also rely on the Smith and Waterman algorithm. These programs are readily utilized with the default parameters recommended by the manufacturer and described in the Wisconsin Sequence Analysis Package referred to above. For example, percent identity of a particular nucleotide sequence to a reference sequence can be determined using the homology algorithm of Smith and Waterman with a default scoring table and a gap penalty of six nucleotide positions.

Alternatively, nucleotide homology can be determined by hybridization of polynucleotides under conditions that form stable duplexes between homologous regions, followed by digestion with single-stranded-specific nuclease(s), and size determination of the digested fragments. DNA sequences that are substantially homologous can be identified in a Southern hybridization experiment under, for example, stringent conditions, as defined for that particular system. Defining appropriate hybridization conditions is within the skill of the art. See, e.g., Sambrook et al. Molecular Cloning: A Laboratory Manual (2d ed., Cold Spring Harbor Laboratory Press, Cold Spring Harbor, N.Y. (1989)).

The term “recombinant” can be used to describe a nucleic acid molecule and refers to a polynucleotide of genomic, RNA, DNA, cDNA, viral, semisynthetic, or synthetic origin which, by virtue of its origin or manipulation is not associated with all or a portion of the polynucleotide with which it is associated in nature.

The term “recombinant” as used with respect to a protein or polypeptide can refer to a polypeptide produced by expression of a recombinant polynucleotide. In general, the gene of interest is cloned and then expressed in transformed organisms, by a method known in the art. The host organism expresses the foreign gene to produce the protein under expression conditions. Disclosed herein is an expression vector comprising a polynucleotide encoding a feline TAS2R polypeptide disclosed herein. In an embodiment, the recombinant vector comprises a polynucleotide consisting of SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, or 25; a polynucleotide consisting of the complement of SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, or 25; or a polynucleotide consisting of a sequence having at least 90%, at least 95%, at least 97%, at least 98%, at least 99% homology with SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, or the complement of SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, or 25. Also disclosed is an expression vector comprising a polynucleotide encoding a chimeric polypeptide disclosed herein.

The term “vector” means a nucleic acid sequence to express a target gene in a host cell. Examples include a plasmid vector, a cosmid vector, a bacteriophage vector, and a viral vector. Examples of viral vectors include a bacteriophage vector, an adenovirus vector, a retrovirus vector, and an adeno-associated virus vector.

For example, the vector may be an expression vector including a membrane targeting or secretion signaling sequence or a leader sequence, in addition to an expression control element such as promoter, operator, initiation codon, termination codon, polyadenylation signal, and enhancer. The vector may be manufactured in various ways known in the art depending on the purpose. An expression vector may include a selection marker for selecting a host cell containing the vector. Further, a replicable expression vector may include an origin of replication.

The term “recombinant vector” or “expression vector” means a vector operably linked to a heterologous nucleotide sequence for the purpose of expression, production, and isolation of the heterologous nucleotide sequence. The heterologous nucleotide sequence can be a nucleotide sequence encoding all or part of a fTAS2R receptor or a chimeric polypeptide disclosed herein.

The recombinant vector may be constructed for use in prokaryotic or eukaryotic host cells. For example, when a prokaryotic cell is used as a host cell, the expression vector used generally includes a strong promoter capable of initiating transcription (for example, pLλ promoter, trp promoter, lac promoter, tac promoter, T7 promoter), a ribosome binding site for initiating translation, and a transcription/translation termination sequence. When a eukaryotic cell is used as a host cell, the vector used generally includes the origin of replication acting in the eukaryotic cell, for example f1 origin of replication, SV40 origin of replication, pMB1 origin of replication, adeno origin of replication, AAV origin of replication, or BBV origin of replication, but is not limited thereto. A promoter in an expression vector for a eukaryotic host cell may be a promoter derived from the genomes of mammalian cells (for example, a metallothionein promoter or an EF-1 alpha promoter) or a promoter derived from mammalian viruses (for example, an adenovirus late promoter, a vaccinia virus 7.5K promoter, a Sindbis promoter, a SV40 promoter, a cytomegalovirus promoter, and a tk promoter of HSV). A transcription termination sequence in an expression vector for a eukaryotic host cell may be, in general, a polyadenylation sequence.

The term “operably linked” refers to a nucleic acid sequence placed into a functional relationship with another nucleic acid sequence. For example, DNA for a presequence or secretory leader is operably linked to DNA for a polypeptide if it is expressed as a preprotein that participates in the secretion of the polypeptide; a promoter or enhancer is operably linked to a coding sequence if it affects the transcription of the sequence; or a ribosome binding site is operably linked to a coding sequence if it is positioned so as to facilitate translation. Generally, “operably linked” means that the DNA sequences being linked are contiguous, and, in the case of a secretory leader, contiguous and in reading phase. However, enhancers do not have to be contiguous. Linking is accomplished by ligation at convenient restriction sites. If such sites do not exist, the synthetic oligonucleotide adaptors or linkers are used in accordance with conventional practice. A nucleic acid is “operably linked” when it is placed into a functional relationship with another nucleic acid sequence. For instance, a promoter or enhancer is operably linked to a coding sequence if it affects the transcription of the sequence. With respect to transcription regulatory sequences, operably linked means that the DNA sequences being linked are contiguous and, where necessary to join two protein coding regions, contiguous and in reading frame.

Further disclosed is a host cell comprising an expression vector or a polynucleotide disclosed herein. A suitable host cell can be transformed with at least one of the recombinant vectors or at least one polynucleotide disclosed herein, for example a polynucleotide consisting of SEQ ID NO:1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, or 25.

The host cell of the vector may be any cell that can be practically utilized by the expression vector. For example, the host cell may be a higher eukaryotic cell, such as a mammalian cell, or a lower eukaryotic cell, such as a yeast cell. Further, the host cell may be a prokaryotic cell, such as a bacterial cell. A prokaryotic host cell may be a Bacillus genus bacterium, such as E. coli JM109, E. coli BL21, E. coli RR1, E. coli LE392, E. coli B, E. coli X 1776, E. coli W3110, Bacillus subtilis, and Bacillus thuringiensis; or an intestinal bacterium, such as Salmonella typhimurium, Serratia marcescens, and various Pseudomonas species. A eukaryotic host cell may be a yeast (e.g., Saccharomyces cerevisiae), an insect cell, a plant cell, or an animal cell, for example, mouse Sp2/0, CHO (Chinese hamster ovary) K1, CHO DG44, PER.C6, W138, BHK, COS-7, 293, HepG2, Huh7, 3T3, RIN, HeLa, HEK-293, or a MDCK cell line.

The polynucleotide or recombinant vector including the polynucleotide may be transferred into the host cell using a method known in the art. For example, when a prokaryotic cell is used as the host cell, the transfer may be performed using a CaCl₂ method or an electroporation method, and when a eukaryotic cell is used as the host cell, the transfer may be performed by microinjection, calcium phosphate precipitation, electroporation, liposome-mediated transfection, LIPOFECTAMINE® (Life Technologies Corporation) transfection, or gene bombardment, but is not limited thereto.

After the expression vector is introduced into the cells, the transfected cells can be cultured under conditions favoring expression of the fTAS2R. The fTAS2R can be recovered from the culture using standard techniques known in the art.

Antibodies to the fTAS2R receptors and the chimeric polypeptides are also disclosed.

An “anti-TAS2R” or a “TAS2R” antibody is an antibody or antibody fragment that specifically binds a polypeptide encoded by a TAS2R gene, cDNA, or a subsequence thereof.

“Antibody” refers to a polypeptide that specifically binds and recognizes an antigen. The term “antibody” or “immunoglobulin,” as used interchangeably herein, includes whole antibodies and any antigen binding fragment (antigen-binding portion) or single chain cognates thereof. An “antibody” comprises at least one heavy (H) chain and one light (L) chain. In naturally occurring IgGs, for example, these heavy and light chains are inter-connected by disulfide bonds and there are two paired heavy and light chains; these two are also inter-connected by disulfide bonds. Each heavy chain is comprised of a heavy chain variable region (abbreviated herein as VH) and a heavy chain constant region. The heavy chain constant region is comprised of three domains, CH1, CH2, and CH3. Each light chain is comprised of a light chain variable region (abbreviated herein as VL) and a light chain constant region. The light chain constant region is comprised of one domain, CL. The VH and VL regions can be further subdivided into regions of hypervariability, termed complementarity determining regions (CDR), interspersed with regions that are more conserved, termed framework regions (FR) or Joining (J) regions (JH or JL in heavy and light chains respectively). Each VH and VL is composed of three CDRs three FRs and a J domain, arranged from amino-terminus to carboxy-terminus in the following order: FR1, CDR1, FR2, CDR2, FR3, CDR3, J. The variable regions of the heavy and light chains bind with an antigen. The constant regions of the antibodies may mediate the binding of the immunoglobulin to host tissues or factors, including various cells of the immune system (e.g., effector cells) or humoral factors such as the first component (Clq) of the classical complement system.

The term “antigen-binding portion” or “antigen-binding fragment” of an antibody, as used herein, refers to one or more fragments of an antibody that retain the ability to specifically bind to an antigen. It has been shown that certain fragments of a full-length antibody can perform the antigen-binding function of an antibody. Examples of binding fragments denoted as an antigen-binding portion or fragment of an antibody include (i) a Fab fragment, a monovalent fragment consisting of the VL, VH, CL and CH1 domains; (ii) a F(ab′)2 fragment, a bivalent fragment comprising two Fab fragments linked by a disulfide bridge at the hinge region; (iii) a Fd fragment consisting of the VH and CH1 domains; (iv) a Fv fragment consisting of the VL and VH domains of a single arm of an antibody, (v) a dAb including VH and VL domains; (vi) a dAb fragment (Ward et al. (1989) Nature 341, 544-546), which consists of a VH domain; (vii) a dAb which consists of a VH or a VL domain; and (viii) an isolated complementarity determining region (CDR) or (ix) a combination of two or more isolated CDRs which may optionally be joined by a synthetic linker. Furthermore, although the two domains of the Fv fragment, VL and VH, are coded for by separate genes, they can be joined, using recombinant methods, by a synthetic linker that enables them to be made as a single protein chain in which the VL and VH regions are paired to form monovalent molecules (such a single chain cognate of an immunoglobulin fragment is known as a single chain Fv (scFv). Such single chain antibodies are also encompassed within the term “antibody fragment.” Antibody fragments are obtained using conventional techniques known to those with skill in the art, and the fragments are screened for utility in the same general manner as are intact antibodies. Antigen-binding fragments can be produced by recombinant DNA techniques, or by enzymatic or chemical cleavage of intact immunoglobulins.

The term “monoclonal antibody” means an antibody obtained from a population of substantially homogeneous antibodies, i.e., the individual antibodies comprising the population are identical except for possible naturally occurring mutations that may be present in minor amounts. In some embodiments, the term “monoclonal antibody” refers to an antibody derived from a single cell clone.

For preparation of monoclonal or polyclonal antibodies, any technique known in the art can be used. Techniques for the production of single chain antibodies (U.S. Pat. No. 4,946,778) can be adapted to produce antibodies to polypeptides disclosed herein. Also, transgenic mice, or other organisms such as other mammals, may be used to express humanized antibodies. Alternatively, phage display technology can be used to identify antibodies and heteromeric Fab fragments that specifically bind to selected antigens.

Methods of production of polyclonal antibodies are known to those of skill in the art. For example, inbred mice (e.g., BALB/C mice) or rabbits are immunized with the fTAS2R, or fragment thereof, using a standard adjuvant, such as Freund's adjuvant, and a standard immunization protocol. The animal's immune response to the immunogen preparation is monitored by taking test bleeds and determining the titer of reactivity to the fTAS2R. When appropriately high titers of antibody to the immunogen are obtained, blood is collected from the animal and antisera are prepared. Further fractionation of the antisera to enrich for antibodies reactive to the protein can be done if desired.

Monoclonal antibodies may be obtained by various techniques familiar to those skilled in the art. Briefly, spleen cells from an animal immunized with a desired antigen are immortalized, commonly by fusion with a myeloma cell. Alternative methods of immortalization include transformation with Epstein Barr Virus, oncogenes, or retroviruses, or other methods well known in the art. Colonies arising from single immortalized cells are screened for production of antibodies of the desired specificity and affinity for the antigen, and yield of the monoclonal antibodies produced by such cells may be enhanced by various techniques, including injection into the peritoneal cavity of a vertebrate host. Alternatively, one may isolate DNA sequences which encode a monoclonal antibody or a binding fragment thereof by screening a DNA library from human B cells according to the general protocol outlined by Huse et al., Science 246:1275-1281 (1989).

Monoclonal antibodies and polyclonal sera are collected and titered against the immunogen protein in an immunoassay, for example, a solid phase immunoassay with the immunogen immobilized on a solid support. Typically, polyclonal antisera with a titer of 104 or greater are selected and tested for their cross reactivity against non-TAS2R proteins, or even other TAS2R family members or other related proteins from other organisms, using a competitive binding immunoassay. Specific polyclonal antisera and monoclonal antibodies will usually bind with a K_(d) of at least about 0.1 mM, more usually at least about 1 μM, specifically at least about 0.1 μM or better, and more specifically 0.01 μM or better.

The term “immunoassay” is an assay that uses an antibody to specifically bind an antigen. The immunoassay is characterized by the use of specific binding properties of a particular antibody to isolate, target, and/or quantify the antigen.

The phrase “specifically (or selectively) binds” to an antibody or “specifically (or selectively) immunoreactive with,” when referring to a protein or peptide, refers to a binding reaction that is determinative of the presence of the protein in a heterogeneous population of proteins and other biologics. Thus, under designated immunoassay conditions, the specified antibodies bind to a particular protein at least two times the background and do not substantially bind in a significant amount to other proteins present in the sample. Specific binding to an antibody under such conditions may require an antibody that is selected for its specificity for a particular protein. For example, polyclonal antibodies raised to a fTAS2R can be selected to obtain only those polyclonal antibodies that are specifically immunoreactive with the fTAS2R protein or an immunogenic portion thereof and not with other proteins, except for orthologs or polymorphic variants and alleles of the TAS2R protein. This selection may be achieved by subtracting out antibodies that cross-react with TAS2R molecules from other species or other TAS2R molecules. Antibodies can also be selected that recognize only fTAS2R GPCR family members but not other GPCRs. A variety of immunoassay formats may be used to select antibodies specifically immunoreactive with a particular protein. For example, solid-phase ELISA immunoassays are routinely used to select antibodies specifically immunoreactive with a protein (see, e.g., Harlow & Lane, Antibodies, A Laboratory Manual (1988)), for a description of immunoassay formats and conditions that can be used to determine specific immunoreactivity). Typically a specific or selective reaction will be at least twice background signal or noise and more typically more than 10 to 100 times background.

Immunoassays can be used to detect, qualitatively or quantitatively, a fTAS2R, e.g., to identify taste receptor cells, especially bitter taste receptor cells, and variants of TAS2R family members.

The anti-fTAS2R antibodies can also be used to isolate feline taste cells from a mixed population of cells obtained from a feline. In an embodiment, isolation of the feline taste cells bound to the anti-fTAS2R antibody can be achieved by flow cytometry. Other methods known in the art can also be used.

Herein a “tastant” means a ligand that can bind to a specific TAS2R receptor or set of receptors. Means to infect/express the nucleic acids and vectors, either individually or as libraries, are well known in the art. A variety of individual cell, organ, or whole animal parameters can be measured by a variety of means. The disclosed fTAS2R sequences can be expressed, for example, in animal taste tissues by delivery with a vector, e.g., adenovirus expression vector.

As known in the art, taste behavior can be determined in a short term assay which directly measures taste preferences by counting licking responses of an animal, e.g., a mouse, using a multi-channel gustometer (e.g., the Davis MS160-Mouse gustometer, DiLog instruments, Tallahassee, Fla.). The mean rate that a mouse will lick a tastant relative to their sampling of an appropriate control (ratio defined as lick rate relative to control) indicates whether the stimulus is appetitive, neutral or aversive. In addition, the change in intake of a palatable stimulus can be evaluated in the presence of the test stimulus to assess enhancement or suppression of the palatable stimulus.

In a further embodiment, animals can be trained to discriminate qualitatively distinct stimuli using operant testing methods known in the art. These animals can then be used to determine qualitative similarity between two stimuli, regardless of palatability or preference.

Alternatively, noninvasive methods such as positron emission tomography (PET) or electroencephalography may be used to monitor neural activity in these preparations. Such preparations may also be used to evaluate the impact of various factors such as age, experience or nutritional state on neural activity elicited by stimuli identified in cell-based experiments to modify receptor function.

Also provided are kits comprising at least one composition, polypeptide, or nucleic acid disclosed herein, optionally contained in a single vial, and may optionally include, e.g., instructions for use in detecting a fTAS2R receptor or a polynucleotide encoding a fTAS2R receptor.

In an embodiment, the kit comprises at least one anti-TAS2R antibody disclosed herein and reagents for detecting a complex between the antibody and the TAS2R antigen. For example, the kit can include a buffer that enables binding reaction between the antibody and the TAS2R antigen in a biological sample, or components for producing the buffer.

The activity of TAS2R polypeptides can be assessed using a variety of in vitro and in vivo assays to determine functional, chemical, and physical effects, e.g., measuring ligand binding (e.g., radioactive ligand binding), second messengers (e.g., cAMP, cGMP, IP3, DAG, or Ca²⁺), ion flux, phosphorylation levels, transcription levels, neurotransmitter levels, and the like. Furthermore, such assays can be used to test for inhibitors and activators of TAS2R family members. Such modulators of taste transduction activity are useful for customizing taste, for example to modify the detection of bitter tastes.

The TAS2R protein of the assay will typically be selected from a polypeptide having a sequence of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26; a conservatively modified variant of SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26; or a sequence that is at least 90%, at least 95%, at least 97%, at least 98%, at least 99% identical to SEQ ID NO:2, SEQ ID NO:4, SEQ ID NO:6, SEQ ID NO:8, SEQ ID NO:10, SEQ ID NO:12, SEQ ID NO:14, SEQ ID NO:16, SEQ ID NO:18, SEQ ID NO:20, SEQ ID NO:22, SEQ ID NO:24, or SEQ ID NO:26.

In some embodiments, the polypeptide of the assays will comprise a domain of a TAS2R protein, such as an extracellular domain, transmembrane region, transmembrane domain, cytoplasmic domain, ligand binding domain, subunit association domain, active site, and the like. Either the TAS2R protein or a domain thereof can be covalently linked to a heterologous protein to create a chimeric protein used in the assays described herein.

Modulators of TAS2R receptor activity are tested using TAS2R polypeptides as described above, either recombinant or naturally occurring. The protein can be isolated, expressed in a cell, expressed in a membrane derived from a cell, expressed in tissue or in an animal, either recombinant or naturally occurring. For example, transformed cells or membranes can be used. Modulation is tested using one of the in vitro or in vivo assays described herein. Taste transduction can also be examined in vitro with soluble or solid state reactions, using a full-length TAS2R-GPCR or a chimeric molecule such as an extracellular domain or transmembrane region, or combination thereof, of a TAS2R receptor covalently linked to a heterologous signal transduction domain, or a heterologous extracellular domain and/or transmembrane region covalently linked to the transmembrane and/or cytoplasmic domain of a TAS2R receptor. Furthermore, ligand-binding domains of the protein of interest can be used in vitro in soluble or solid state reactions to assay for ligand binding. In numerous embodiments, a chimeric receptor will be made that comprises all or part of a TAS2R polypeptide as well an additional sequence that facilitates the localization of the TAS2R to the membrane, such as rhodopsin, e.g., an N-terminal fragment of a rhodopsin protein.

Ligand binding to a TAS2R protein, a domain, or chimeric protein can be tested in solution, in a bilayer membrane, attached to a solid phase, in a lipid monolayer, or in vesicles. Binding of a modulator can be tested using, e.g., changes in spectroscopic characteristics (e.g., fluorescence, absorbance, refractive index), hydrodynamic (e.g., shape), chromatographic, or solubility properties.

Receptor-G-protein interactions can also be examined. For example, binding of the G-protein to the receptor or its release from the receptor can be examined. For example, in the absence of GTP, an activator will lead to the formation of a tight complex of a G protein (all three subunits) with the receptor. This complex can be detected in a variety of ways, as noted above. Such an assay can be modified to search for inhibitors, e.g., by adding an activator to the receptor and G protein in the absence of GTP, which form a tight complex, and then screen for inhibitors by looking at dissociation of the receptor-G protein complex. In the presence of GTP, release of the alpha subunit of the G protein from the other two G protein subunits serves as a criterion of activation.

In some embodiments, TAS2R-Gustducin interactions are monitored as a function of TAS2R receptor activation. Ligand dependent coupling of TAS2R receptors with Gustducin can be used as a marker to identify modifiers of any member of the TAS2R family.

An activated or inhibited G-protein will in turn alter the properties of target enzymes, channels, and other effector proteins. The classic examples are the activation of cGMP phosphodiesterase by transducin in the visual system, adenylate cyclase by the stimulatory G-protein, phospholipase C by Gq and other cognate G proteins, and modulation of diverse channels by Gi and other G proteins. Downstream consequences can also be examined such as generation of diacyl glycerol and IP3 by phospholipase C, and in turn, for calcium mobilization by IP3.

In an embodiment, a TAS2R polypeptide is expressed in a eukaryotic cell as a chimeric receptor with a heterologous, chaperone sequence that facilitates its maturation, targeting through the secretory pathway or membrane localization. In a preferred embodiment, the heterologous sequence is a rhodopsin sequence, such as an N-terminal fragment of a rhodopsin. Such chimeric TAS2R receptors can be expressed in any eukaryotic cell, such as ATCC #CRL-11268 cells. Preferably, the cells comprise a functional G protein, e.g., G[alpha]15, that is capable of coupling the chimeric receptor to an intracellular signaling pathway or to a signaling protein such as phospholipase Cβ. Activation of such chimeric receptors in such cells can be detected using any standard method, such as by detecting changes in intracellular calcium by detecting FURA-2 dependent fluorescence in the cell.

Activated GPCR proteins become substrates for kinases that phosphorylate the C-terminal tail of the receptor (and possibly other sites as well). Thus, activators will promote the transfer of ³²P from gamma-labeled GTP to the receptor, which can be assayed with a scintillation counter. The phosphorylation of the C-terminal tail will promote the binding of arrestin-like proteins and will interfere with the binding of G-proteins. The kinase/arrestin pathway plays a key role in the desensitization of many GPCR proteins. For example, compounds that modulate the duration a taste receptor stays active would be useful as a means of prolonging a desired taste or cutting off an unpleasant one.

Samples or assays that are treated with a potential TAS2R protein inhibitor or activator are compared to control samples without the test compound, to examine the extent of modulation. Such assays may be carried out in the presence of a bitter tastant that is known to activate the particular receptor, and modulation of the bitter-tastant-dependent activation monitored. Control samples (untreated with activators or inhibitors) are assigned a relative TAS2R activity value of 100. Inhibition of a TAS2R protein is achieved when the TAS2R activity value relative to the control is about 90%, optionally 50%, optionally 25-0%. Activation of a TAS2R protein is achieved when the TAS2R activity value relative to the control is 110%, optionally 150%, 200-500%, or 1000-2000%.

Changes in ion flux may be assessed by determining changes in polarization (i.e., electrical potential) of the cell or membrane expressing a TAS2R protein. One means to determine changes in cellular polarization is by measuring changes in current (thereby measuring changes in polarization) with voltage-clamp and patch-clamp techniques, e.g., the “cell-attached” mode, the “inside-out” mode, and the “whole cell” mode. Whole cell currents are conveniently determined using standard methodology known in the art. Other known assays include: radio labeled ion flux assays and fluorescence assays using voltage-sensitive dyes. Generally, the compounds to be tested are present in the range from 1 pM to 100 mM.

The effects of the test compounds upon the function of the polypeptides can be measured by examining any of the parameters described above. Any suitable physiological change that affects GPCR activity can be used to assess the influence of a test compound on the polypeptides disclosed herein. When the functional consequences are determined using intact cells or animals, one can also measure a variety of effects such as transmitter release, hormone release, transcriptional changes to both known and uncharacterized genetic markers (e.g., Northern blots), changes in cell metabolism such as cell growth or pH changes, and changes in intracellular second messengers such as Ca²⁺, IP3, cGMP, or cAMP.

In an embodiment, assays for G-protein coupled receptors include cells that are loaded with ion or voltage sensitive dyes to report receptor activity. Assays for determining activity of such receptors can also use known agonists and antagonists for other G-protein coupled receptors as negative or positive controls to assess activity of tested compounds. In assays for identifying modulatory compounds (e.g., agonists, antagonists), changes in the level of ions in the cytoplasm or membrane voltage will be monitored using an ion sensitive or membrane voltage fluorescent indicator, respectively. Ion-sensitive indicators and voltage probes that may be employed are commercially available from a variety of sources. For G-protein coupled receptors, promiscuous G-proteins such as Gα15 and Gα16 can be used in the assay of choice. Such promiscuous G-proteins allow coupling of a wide range of receptors.

Receptor activation typically initiates subsequent intracellular events, e.g., increases in second messengers such as IP3, which releases intracellular stores of calcium ions. Activation of some G-protein coupled receptors stimulates the formation of inositol triphosphate (IP3) through phospholipase C-mediated hydrolysis of phosphatidylinositol (Berridge & Irvine, Nature 312:315-21 (1984)). IP3 in turn stimulates the release of intracellular calcium ion stores. Thus, a change in cytoplasmic calcium ion levels, or a change in second messenger levels such as IP3 can be used to assess G-protein coupled receptor function. Cells expressing such G-protein coupled receptors may exhibit increased cytoplasmic calcium levels as a result of contribution from both intracellular stores and via activation of ion channels, in which case it may be desirable although not necessary to conduct such assays in calcium-free buffer, optionally supplemented with a chelating agent such as EGTA, to distinguish fluorescence response resulting from calcium release from internal stores.

Other assays can involve determining the activity of receptors which, when activated, result in a change in the level of intracellular cyclic nucleotides, e.g., cAMP or cGMP, by activating or inhibiting enzymes such as adenylate cyclase. There are cyclic nucleotide-gated ion channels, e.g., rod photoreceptor cell channels and olfactory neuron channels that are permeable to cations upon activation by binding of cAMP or cGMP (see, e.g., Altenhofen et al., Proc. Natl. Acad. Sci. U.S.A. 88:9868-9872 (1991) and Dhallan et al., Nature 347:184-187 (1990)). In cases where activation of the receptor results in a decrease in cyclic nucleotide levels, it may be preferable to expose the cells to agents that increase intracellular cyclic nucleotide levels, e.g., forskolin, prior to adding a receptor-activating compound to the cells in the assay. Cells for this type of assay can be made by co-transfection of a host cell with DNA encoding a cyclic nucleotide-crated ion channel, GPCR phosphatase and DNA encoding a receptor (e.g., certain glutamate receptors, muscarinic acetylcholine receptors, dopamine receptors, serotonin receptors, and the like), which, when activated, causes a change in cyclic nucleotide levels in the cytoplasm.

In an embodiment, TAS2R protein activity is measured by expressing a TAS2R gene in a heterologous cell with a promiscuous G-protein that links the receptor to a phospholipase C signal transduction pathway (see Offermanns & Simon, J. Biol. Chem. 270:15175-15180 (1995)). Optionally the cell line is a eukaryotic cell line which does not naturally express TAS2R genes and the promiscuous G-protein is Gα15 (Offermanns & Simon, supra). Modulation of taste transduction is assayed by measuring changes in intracellular Ca²⁺ levels, which change in response to modulation of the TAS2R signal transduction pathway via administration of a molecule that associates with a TAS2R protein. Changes in Ca²⁺ levels are optionally measured using fluorescent Ca²⁺ indicator dyes and fluorometric imaging.

In one embodiment, the changes in intracellular cAMP or cGMP can be measured using immunoassays. The method described in Offermanns & Simon, J. Biol. Chem. 270:15175-15180 (1995) may be used to determine the level of cAMP. Also, the method described in Felley-Bosco et al., Am. J. Resp. Cell and Mol. Biol. 11:159-164 (1994) may be used to determine the level of cGMP. Further, an assay kit for measuring cAMP and/or cGMP is described in U.S. Pat. No. 4,115,538, herein incorporated by reference.

In another embodiment, phosphatidyl inositol (PI) hydrolysis can be analyzed according to U.S. Pat. No. 5,436,128, herein incorporated by reference. Briefly, the assay involves labeling of cells with 3H-myoinositol for 48 or more hours. The labeled cells are treated with a test compound for one hour. The treated cells are lysed and extracted in chloroform-methanol-water after which the inositol phosphates were separated by ion exchange chromatography and quantified by scintillation counting. Fold stimulation is determined by calculating the ratio of cpm in the presence of agonist to cpm in the presence of buffer control. Likewise, fold inhibition is determined by calculating the ratio of cpm in the presence of antagonist to cpm in the presence of buffer control (which may or may not contain an agonist).

In another embodiment, transcription levels can be measured to assess the effects of a test compound on signal transduction. A host cell containing a TAS2R protein of interest is contacted with a test compound for a sufficient time to effect any interactions, and then the level of gene expression is measured. The amount of time to effect such interactions may be empirically determined, such as by running a time course and measuring the level of transcription as a function of time. The amount of transcription may be measured by using any method known to those of skill in the art to be suitable. For example, mRNA expression of the protein of interest may be detected using Northern blots or their polypeptide products may be identified using immunoassays. Alternatively, transcription based assays using reporter gene may be used as described in U.S. Pat. No. 5,436,128, herein incorporated by reference. The reporter genes can be, e.g., chloramphenicol acetyltransferase, luciferase, β-galactosidase and alkaline phosphatase. Furthermore, the protein of interest can be used as an indirect reporter via attachment to a second reporter such as green fluorescent protein (see, e.g., Mistili & Spector, Nature Biotechnology 15:961-964 (1997)).

The amount of transcription is then compared to the amount of transcription in either the same cell in the absence of the test compound, or it may be compared with the amount of transcription in a substantially identical cell that lacks the protein of interest. A substantially identical cell may be derived from the same cells from which the recombinant cell was prepared but which had not been modified by introduction of heterologous DNA. Any difference in the amount of transcription indicates that the test compound has in some manner altered the activity of the protein of interest.

The compounds tested as modulators of a TAS2R family member can be any small chemical compound, or a biological entity, such as a protein, sugar, nucleic acid or lipid. Alternatively, modulators can be genetically altered versions of a TAS2R gene. Typically, test compounds will be small chemical molecules and peptides. Essentially any chemical compound can be used as a potential modulator or ligand in the assays and methods disclosed herein, although most often compounds can be dissolved in aqueous or organic (especially DMSO-based) solutions are used. The assays are designed to screen large chemical libraries by automating the assay steps and providing compounds from any convenient source to assays, which are typically run in parallel (e.g., in microtiter formats on microtiter plates in robotic assays). There are many suppliers of chemical compounds, including Sigma-Aldrich (St. Louis, Mo.), Fluka Chemika-Biochemica Analytika (Buchs, Switzerland) and the like.

Methods to identify compounds that bind and/or modulate fTAS2R receptors are disclosed. A“TAS2R binding partner” is a compound that directly or indirectly binds a TAS2R polypeptide disclosed herein. In an embodiment, the method comprises contacting TAS2R receptor with a test compound suspected of binding TAS2R receptor; and detecting binding between the compound and the TAS2R receptor. In one variation, a composition comprising a cell expressing TAS2R receptor on its surface is used in the method. In another variation, isolated TAS2R receptor or cell membranes comprising TAS2R receptor are employed. The binding may be measured directly, e. g., by using a labeled compound, or may be measured indirectly. Compounds identified as binding a TAS2R receptor may be further tested in other assays including TAS2R activity assays and/or in vivo models, in order to confirm or quantitate their activity.

“Substantially the same” biological activity refers to a polypeptide fragment, derivative, homolog, analog, or variant retaining at least about 50%, 55%, 60%, 65%, 70%, preferably at least about 75%, 80%, 85%, 90%, more preferably at least about 91%, 92%, 93%, 94%, 95%, and most preferably at least about 96%, 97%, 98%, 99% or greater biological activity of the parent polypeptide. The extent to which a polypeptide fragment, derivative, homolog, analog, or variant retains the biological activity of the parent polypeptide may be assessed by any means available in the art, including, but not limited to, the assays listed or described herein.

Specific binding molecules, including natural ligands and synthetic compounds, can be identified or developed using isolated or recombinant TAS2R products, TAS2R variants, or cells expressing such products. Binding partners are useful for purifying TAS2R products and detection or quantification of TAS2R products in fluid and tissue samples using known immunological procedures. Binding molecules are also useful in modulating (i.e., blocking, inhibiting or stimulating) biological activities of TAS2R, especially those activities involved in signal transduction. Binding molecules also are useful in methods for predicting the taste perception of an organism such as a mammal by detecting a TAS2R polypeptide in a biological sample of a feline.

The term “taste perception” as used herein refers to a response (e.g., biochemical, behavioral) or sensitivity of a TAS2R receptor to a taste stimulus. Modification of taste perception includes an alteration of (enhancement of, reduction of, or change to) a biochemical response, an ingestive response, a taste preference, or a general behavior of a mammal in response to a compound. “Taste perception” does not require, though it can include, transmission of a neural signal resulting in the in vivo sensation of taste by a mammal.

The nucleic acid and amino acid sequence information disclosed herein also makes possible identification of binding partner compounds with which a TAS2R polypeptide or polynucleotide will interact. Methods to identify binding partner compounds include solution assays, in vitro assays wherein TAS2R polypeptides are immobilized, and cell-based assays.

Methods for identifying TAS2R-binding partners are disclosed herein. In solution assays, the methods can comprise the steps of (a) contacting a TAS2R receptor with one or more candidate binding partner compounds and (b) identifying the compounds that bind to the TAS2R receptor. Identification of the compounds that bind the TAS2R receptor can be achieved by isolating the TAS2R polypeptide/binding partner complex, and separating the binding partner compound from the TAS2R polypeptide. In one aspect, the TAS2R polypeptide/binding partner complex is isolated using an antibody immunospecific for either the TAS2R receptor or the candidate binding partner compound.

In still other embodiments, either the TAS2R receptor or the candidate binding partner compound comprises a label or tag that facilitates its isolation, and methods to identify binding partner compounds include a step of isolating the TAS2R polypeptide/binding partner complex through interaction with the label or tag. An exemplary tag of this type is a poly-histidine sequence, generally around six histidine residues, that permits isolation of a compound so labeled using nickel chelation. Other labels and tags, such as the FLAG tag (Eastman Kodak, Rochester, N.Y.), are well known and routinely used in the art.

In one variation of an in vitro assay, the method comprises the steps of (a) contacting an immobilized TAS2R receptor with a candidate binding partner compound and (b) detecting binding of the candidate compound to the TAS2R receptor. In an alternative embodiment, the candidate binding partner compound is immobilized and binding of TAS2R receptor is detected. Immobilization is accomplished using any of the methods well known in the art, including covalent bonding to a support, a bead, or a chromatographic resin, as well as non-covalent, high affinity interactions such as antibody binding, or use of streptavidin/biotin binding wherein the immobilized compound includes a biotin moiety. The support may, for example, be formulated into a feline-specific electronic tongue or biosensor.

Detection of binding can be accomplished (i) using a radioactive label on the compound that is not immobilized, (ii) using a fluorescent label on the non-immobilized compound, (iii) using an antibody immunospecific for the non-immobilized compound, (iv) using a label on the non-immobilized compound that excites a fluorescent support to which the immobilized compound is attached, as well as other techniques well known and routinely practiced in the art.

In another embodiment, cell-based assays are used to identify binding partner compounds of a TAS2R receptor. In one embodiment, the method comprises the steps of contacting a TAS2R receptor expressed on the surface of a cell with a candidate binding partner compound and detecting binding of the candidate binding partner compound to the TAS2R receptor. In some embodiments, the detection comprises detecting physiological event in the cell caused by the binding of the molecule.

Methods of identifying compounds that bind to either TAS2R receptor or nucleic acid molecules encoding TAS2R receptor are also disclosed. In an embodiment, the method comprises contacting TAS2R receptor, or a nucleic acid molecule encoding the same, with a compound, and determining whether the compound binds TAS2R receptor or a nucleic acid molecule encoding the same. Binding can be determined by any binding assay known to the skilled artisan, including gel-shift assays, Western blots, radiolabeled competition assay, phage-based expression cloning, co-fractionation by chromatography, co-precipitation, cross-linking, interaction trap/two-hybrid analysis, southwestern analysis, and ELISA.

The methods may also use ligands that are attached to a label, such as a radiolabel (e.g., ¹²⁵I, ³⁵S, ³²P, ³³P, ³H), a fluorescence label, a chemiluminescent label, an enzymic label, and an immunogenic label.

Modulators can include non-peptide molecules such as non-peptide mimetics, non-peptide allosteric effectors, and peptides.

The TAS2R polypeptide or polynucleotide employed in such a test may either be free in solution, attached to a solid support, borne on a cell surface, located intracellularly, or associated with a portion of a cell. One skilled in the art can, for example, measure the formation of complexes between TAS2R receptor or polynucleotide and the compound being tested. Alternatively, one skilled in the art can examine the diminution in complex formation between TAS2R receptor or polynucleotide and its substrate caused by the compound being tested. In some embodiments, the recognition sites of the TAS2R receptor or polynucleotide are coupled with a monitoring system, either electrical or optical. An appropriate chemical stimulus can bind to the receptor's ligand binding domain, changing the receptor conformation to a degree that the coupled electronics or optical changes can be observed on a read-out. In an embodiment, the solid support is formulated into a feline-specific electronic tongue or biosensor.

The term “solid support” refers to a material or group of materials having a rigid or semi-rigid surface or surfaces. Examples of materials include plastics (e.g., polycarbonate), complex carbohydrates (e.g., agarose and sepharose), acrylic resins (e.g., polyacrylamide and latex beads), nitrocellulose, glass, silicon wafers, and positively charged nylon. In some aspects, at least one surface of the solid support can be substantially flat, although in some aspects it may be desirable to physically separate regions for different molecules with, for example, wells, raised regions, pins, etched trenches, or the like. In certain aspects, the solid support(s) will take the form of beads, resins, gels, microspheres, or other geometric configurations.

In another embodiment, high throughput screening (HTS) for compounds having suitable binding affinity to TAS2R receptor is employed. Briefly, large numbers of different small peptide test compounds are synthesized on a solid substrate. The peptide test compounds are contacted with TAS2R receptor and washed. Bound TAS2R receptor is then detected by methods well known in the art. Purified polypeptides of the invention can also be coated directly onto plates for use in the aforementioned drug screening techniques. In addition, non-neutralizing antibodies can be used to capture the protein and immobilize it on the solid support.

Generally, an expressed TAS2R receptor can be used for HTS binding assays in conjunction with a ligand, such as an amino acid or carbohydrate. The identified peptide is labeled with a suitable radioisotope, including, ¹²¹I, ³H, ³⁵S or ³²P, by methods that are well known to those skilled in the art. Alternatively, the peptides may be labeled by well-known methods with a suitable fluorescent derivative (Baindur et al., Drug Dev. Res., 1994, 33, 373-398; Rogers, Drug Discovery Today, 1997, 2, 156-160). Radioactive ligand specifically bound to the receptor in membrane preparations made from the cell line expressing the recombinant protein can be detected in HTS assays in one of several standard ways, including filtration of the receptor-ligand complex to separate bound ligand from unbound ligand. Alternative methods include a scintillation proximity assay (SPA) or a FlashPlate format in which such separation is unnecessary. Binding of fluorescent ligands can be detected in various ways, including fluorescence energy transfer (FRET), direct spectrophotofluorometric analysis of bound ligand, or fluorescence polarization.

Other assays may be used to identify specific ligands of a TAS2R receptor, including assays that identify ligands of the target protein through measuring direct binding of test ligands to the target, as well as assays that identify ligands of target proteins through affinity ultrafiltration with ion spray mass spectroscopy/HPLC methods or other physical and analytical methods. Alternatively, such binding interactions are evaluated indirectly using the yeast two-hybrid system, a genetic assay for detecting interactions between two proteins or polypeptides.

Also disclosed are methods of identifying compounds that modulate (i. e., increase or decrease) activity of TAS2R receptor comprising contacting TAS2R receptor with a compound, and determining whether the compound modifies activity of TAS2R receptor. The activity in the presence of the test compound is compared to the activity in the absence of the test compound. Where the activity of the sample containing the test compound is higher than the activity in the sample lacking the test compound, the compound is an agonist. Similarly, where the activity of the sample containing the test compound is lower than the activity in the sample lacking the test compound, the compound is an antagonist.

Agents that modulate (i.e., increase, decrease, or block) TAS2R receptor activity or expression also may be identified, for example, by incubating a putative modulator with a cell containing a TAS2R polypeptide or polynucleotide and determining the effect of the putative modulator on TAS2R receptor activity or expression. The selectivity of a compound that modulates the activity of TAS2R receptor can be evaluated by comparing its effects on TAS2R receptor to its effect on other TAS2R receptors. Selective modulators may include, for example, antibodies and other proteins, peptides, or organic molecules that specifically bind to a TAS2R polypeptide or a TAS2R receptor-encoding nucleic acid. Compounds identified as modulating TAS2R receptor activity may be further tested in other assays including in vivo models, in order to confirm or quantitate their activity.

TAS2R polynucleotides and polypeptides, and their homologs, are useful tools for identifying taste receptor expressing cells, and for examining taste transduction. TAS2R family member-specific reagents that specifically hybridize to TAS2R nucleic acids, such as TAS2R probes and primers, and TAS2R specific reagents that specifically bind to a TAS2R protein, e.g., TAS2R antibodies are used to examine taste cell expression and taste transduction regulation. For example, a TAS2R antibody can be used to identify and/or isolate feline taste cells expressing the particular TAS2R from a mixed feline cell population. For example, polynucleotide probes disclosed herein may be used in tissue distribution studies and diagnostic assays.

Nucleic acid assays for the presence of DNA and RNA for a TAS2R family member in a sample include numerous techniques known to those skilled in the art, such as Southern analysis, Northern analysis, dot blots, RNase protection, S1 analysis, amplification techniques such as polymerase chain reaction (PCR) and ligase chain reaction (LCR), and in situ hybridization. In addition, a TAS2R protein can be detected with the various immunoassay techniques known in the art. The test sample is typically compared to both a positive control (e.g., a sample expressing a recombinant TAS2R protein) and a negative control.

Also provided are kits for screening for modulators of TAS2R family members. Such kits can be prepared from readily available materials and reagents. For example, such kits can comprise any one or more of the following materials: TAS2R nucleic acids or proteins, reaction tubes, and instructions for testing TAS2R activity. Optionally, the kit contains a biologically active TAS2R receptor. A wide variety of kits and components can be prepared, depending upon the intended user of the kit and the particular needs of the user.

Other embodiments of the present invention are described in the following non-limiting Examples.

EXAMPLES Example 1 Determining Feline Bitter Taste Receptor (TAS2R) Gene and Polypeptide Sequences

In this example, feline TAS2R genes were identified, by querying the NCBI Felis catus whole genome shotgun contigs database with human bitter receptor gene sequences. Human gene sequences used are identified by NCBI Gene IDs in Table 2.

TABLE 2 NCBI Gene IDs for all functional and pseudogene hTAS2Rs used to identify feline bitter genes. Human Bitter Receptor gene Gene ID Functional Genes TAS2R1 (TAS2R1; TRB7) 50834 TAS2R3 (TAS2R3) 50831 TAS2R4 (TAS2R4) 50832 TAS2R5 (TAS2R5) 54429 TAS2R7 (TAS2R7; TRB4) 50837 TAS2R8 (TAS2R8; TRB5) 50836 TAS2R9 (TAS2R9; TRB6) 50835 TAS2R10 (TRB2; TAS2R10) 50839 TAS2R13 (TRB3; TAS2R13) 50838 TAS2R14 (TRB1; TAS2R14) 50840 TAS2R16 (TAS2R16) 50833 TAS2R19 (TAS2R19; TAS2R23; 259294 TAS2R48; MSTP058; TAS2R23; TAS2R48) TAS2R20 (TAS2R20; TAS2R49; 259295 TAS2R56; TAS2R49) TAS2R30 (TAS2R30; TAS2R47; 259293 TAS2R47) TAS2R31 (TAS2R31; TAS2R44; 259290 TAS2R53; TAS2R44) TAS2R38 (PTC; TAS2R38; TAS2R61) 5726 TAS2R39 (TAS2R39; TAS2R57) 259285 TAS2R40 (GPR60; TAS2R40; 259286 TAS2R58) TAS2R41 (TAS2R41; TAS2R59) 259287 TAS2R42 (TAS2R24; TAS2R55; 353164 hTAS2R55; TAS2R55) TAS2R43 (TAS2R43; TAS2R52) 259289 TAS2R45 (GPR59; TAS2R45; ZG24P) 259291 TAS2R46 (TAS2R46; TAS2R54) 259292 TAS2R50 (TAS2R50; TAS2R51; 259296 TAS2R51) TAS2R60 (TAS2R56; TAS2R60) 338398 Pseudogenes TAS2R2P (PS9; TAS2R2; TAS2R02; 338396 TAS2R2) TAS2R12P (PS10; TAS2R12; 266656 TAS2R12; TAS2R26) TAS2R15P (PS8; TAS2R15) 266657 TAS2R18 (PS4; TAS2R18; 338414 TAS2R65; TAS2R65; TAS2R65P) TAS2R62P (PS1; TAS2R62; 338399 TAS2R62) TAS2R63P (PS6; TAS2R63) 338413 TAS2R64P (PS2; TAS2R64; 338412 TAS2R64P) TAS2R67P (PS5) 448991 TAS2R68P (PS7; TAS2R68P) 100653053

Individual contigs among the hits were downloaded for manual identification of start (ATG) and stop (TAA, TGA, or TAG) codons and to determine if the gene is likely full length. When sequences from both feline genome assemblies were obtained, they were compared.

Predicted functional genes were identified based on a set of rules selected to include a protein which is approximately 300 amino acids in length, the start site and stop site are in similar locations as the human protein when the blasted sequences are aligned, then the sequence was compared to the sequence of the orthologous canine bitter gene to verify that similarity was reasonable. Table 3 identifies canine bitter gene sequences used.

TABLE 3 NCBI Gene IDs for all functional and pseudogene canine TAS2Rs used Canine Bitter Receptor gene Gene ID Functional Genes 1 TAS2R1 (CAFA-TAS2R1) 100271742 2 Cafa-TAS2R2 100271741 3 TAS2R3 (CAFA-TAS2R3) 100271736 4 TAS2R4 100688996 5 TAS2R5 (CAFA-TAS2R5) 100271743 6 TAS2R7 (CAFA-TAS2R7) 100271739 7 TAS2R10 (CAFA-TAS2R10) 100271734 8 Cafa-TAS2R12 100271738 9 TAS2R38 (CAFA-TAS2R38) 100271737 10 TAS2R39 (CAFA-TAS2R39) 100271735 11 TAS2R40 608842 12 TAS2R41 482734 13 TAS2R42 (CAFA-TAS2R55) 100271731 14 Cafa-TAS2R43 100271744 15 TAS2R62-like 608741 16 Cafa-TAS2R67 100271740 Pseudogenes 1 TAS2R8P 100682910 2 TAS2R9P 100686911 3 Cafa-TAS2R44P GenBank: AB249699.1 4 TAS2R46-like 100682759 5 TAS2R60-like 100856773 6 TAS2R104-like 100682833

Table 4 below summarizes the full length feline genes identified. The % protein similarity between the feline gene and closest human homologue is presented in the table.

TABLE 4 Full length Feline Bitter Receptor Genes Identified Predicted Feline Functional human Best % similarity to Gene homologue human sequence TAS2R1 yes 60.5% TAS2R2 NO 74.8% TAS2R3 yes 74.4% TAS2R4 yes 71.9% TAS2R7 yes 74.4% TAS2R9 yes 68.3% TAS2R10 yes 67.8% TAS2R12 NO 51.0% TAS2R38 yes 67.6% TAS2R42 NO 56.1% TAS2R43 yes 59.0% TAS2R44 yes 59.9% TAS2R67 NO 47.6%

Cloning of each of the feline bitter genes to confirm the DNA sequence was performed after amplifying the desired gene by polymerase chain reaction (PCR) using the genomic DNA of a single cat. Potential primers to amplify each feline gene were designed using commercial software. Sets of primers were selected from among those designed based on predicted annealing temperature, fidelity, potential for dimerization and mispriming, and location of the desired sequence in order to amplify the feline gene sequence and determine the DNA sequence from isolated feline genomic DNA.

The process of amplification and cloning of a representative gene, TAS2R38, is briefly described. The fTAS2R38 sequence was amplified via PCR using Easy A High Fidelity PCR Cloning Enzyme (Agilent, Santa Clara Calif.), custom primers, and feline genomic DNA as a template.

The resulting PCR product was ligated into the pGEM-T Easy Vector (Promega, Madison Wis.). DH5-α bacterial cells (Life Technologies; Carlsbad, Calif.) were transformed with the vector. Plasmids were purified from cultures of the transformed DH5-alpha cells using the Plasmid Miniprep Kit (Omega BioTec, Norcross, Ga.). Sequencing of the gene using the purified plasmid DNA was performed by the Core DNA Sequencing Facility at the University of Illinois, Champaign-Urbana. The sequencing data was analyzed with SeqMan Pro (DNAStar, Madison Wis.) to determine the quality of the data and to edit the data.

The gene sequence determined from the isolated feline genomic DNA sequencing was compared against the sequences obtained from the whole genome shotgun contigs and analyzed to identify specific nucleotide differences, predicted protein sequence, and protein structure. Sequences disclosed in the sequence listing for each of the feline bitter taste receptor gene cDNAs and polypeptides are identified by the SEQ ID NOs shown in Table 5.

TABLE 5 SEQ ID NOs of feline bitter taste receptor gene cDNA and polypeptide sequences SEQ ID NO. Feline TAS2R Sequence 1 R1 cDNA 2 R1 polypeptide 3 R2 cDNA 4 R2 polypeptide 5 R3 cDNA 6 R3 polypeptide 7 R4 cDNA 8 R4 polypeptide 9 R7 cDNA 10 R7 polypeptide 11 R9 cDNA 12 R9 polypeptide 13 R10 cDNA 14 R10 polypeptide 15 R12 cDNA 16 R12 polypeptide 17 R38 cDNA 18 R38 polypeptide 19 R42 cDNA 20 R42 polypeptide 21 R43 cDNA 22 R43 polypeptide 23 R44 cDNA 24 R44 polypeptide 25 R67 cDNA 26 R67 polypeptide

In general, the feline gene is named after its homologous human counterpart, as shown in Table 6. However for a feline gene similar to many human genes, such as fTAS2R43, the feline gene is named as its homologous canine counterpart.

TABLE 6 Corresponding genes in felines, canines and humans Predicted Predicted Feline Gene Canine Gene Human Gene TAS2R1 TAS2R1 TAS1R1 TAS2R2 CAFA-T2R2 TAS2R2P TAS2R3 TAS2R3 TAS2R3 TAS2R4 TAS2R4 TAS2R4 TAS2R5P TAS2R5 TAS2R5 TAS2R7 TAS2R7 TAS2R7 TAS2R8P TAS2R8P TAS2R8 TAS2R9 TAS2R9P TAS2R9 TAS2R10 TAS2R10 TAS2R10 TAS2R12 TAS2R12 TAS2R12P TAS2R16P N/A TAS2R16 TAS2R38 TAS2R38 TAS2R38 TAS2R39P TAS2R39 TAS2R39 TAS2R40P TAS2R40 TAS2R40 TAS2R41P TAS2R41 TAS2R41 TAS2R42 TAS2R42 hTAS2R42, 18P, 67P TAS2R43 TAS2R43 hTAS2R13, 14, 19, 20, 30, 31,43, 45, 46, 50, 15P, 63P, 64P, 68P TAS2R44 CAFA-T2R44P hTAS2R13, 14, 19, 20, 30, 31, 43, 45, 46, 50, 15P, 63P, 64P, 68P TAS2R67 CAFA-T2R67 hTAS2R42, 18P, 67P TAS2R60P Tas2R60P-like TAS2R60 TAS2R62P TAS2R62-like TAS2R62P

A sequence alignment of the 3rd through the 7th transmembrane (TM) regions of several human and feline bitter receptors is shown in FIG. 1. The sequence alignment illustrates the substantial degree of homology of this region in bitter taste receptors of the two species.

A sequence alignment of human TAS2R38 polypeptide (SEQ ID NO.: 31) and feline TAS2R38 polypeptide (SEQ ID NO.: 18) determined from sequencing genomic DNA of five individual cats is shown in FIG. 2. Amino acids in hTAS2R38 that differ from those in fTAS2R38 are boxed in FIG. 2. The positions of the human polymorphisms known to affect taste perception of 6-n-propylthiouracil (PROP), A49P, V262A, I293V (where AVI is a non-taster and PAV is a taster) are shaded grey in FIG. 2. The residues known to be important for phenylthiocarbamide (PTC) binding to the human TAS2R38 receptor are denoted in FIG. 2 by a thick black box (residues 99-100, 103, 255, and 259) These amino acids either directly bind PTC, contribute to the binding pocket, or are involved in receptor activation by associating with other amino acids.

TOPCONS was used to identify the seven transmembrane regions and the extracellular and cytoplasmic loops of each fTAS2R polypeptide. Results of this analysis are presented in Table 1.

Example 2 Expression Systems for Feline TAS2R

A. Generation of Expression Vectors for Feline TAS2R

This example describes generation of an expression vector for a representative feline bitter receptor, TAS2R38.

The full length gene of feline TAS2R38 was amplified by polymerase chain reaction (PCR) using gene-specific primers that span the entire coding region.

The TAS2R38 cDNA was subcloned into an expression cassette based on the plasmid/expression vector pcDNA3.1D-V5His (Life Technologies, Carlsbad, Calif., US), which contains within its multiple cloning sites the nucleotide sequence coding for the FLAG epitope to allow surface detection of the receptor, then the first 45 amino acids of the rat somatostatin receptor subtype 3 (RSS tag) to facilitate cell surface targeting of the transgene, and the nucleotide sequence coding for the herpes simplex virus (HSV) glycoprotein D epitope (HSV epitope) for facilitating immunocytochemical detection (HSV Tag) on the carboxy terminus.

The nucleic acid sequences encoding FLAG, the RSS tag, TAS2R38, and the HSV tag are fused, in that order, in frame to create a construct to allow translation into the receptor protein. The resulting receptor cDNA in the expression vector encodes the joined amino acid sequences of TAS2R38 preceded by the RSS tag and followed by the HSV tag.

The expression vector including the construct is called pcDNA3.11D-FLAGV5His-TAS2R38 and allows for expression of the TAS2R38 protein (SEQ ID NO:18).

Generation of an expression vector for the other fTAS2Rs disclosed herein is analogous. The restriction enzymes used are adapted accordingly.

B. Generation of Cell Lines Transiently Expressing fTAS2R

Cell lines that transiently express a desired TAS2R disclosed herein are generated by transfecting the appropriate expression vector, e.g., pcDNA3.1 D-FLAGV5His-TAS2R38, constructed as described above in Ex. 2A into cells of a eukaryotic cell line (e.g., ATCC #CRL-11268 or Life Technologies, Cat #R700-07).

On day 0, 60,000 cells per well are plated on poly lysine coated, black 96 well plates with clear bottoms (Costar). The following day the cells are transfected with 150 ng TAS2R38 expression vector, e.g., pcDNA3.1D-FLAGV5His (Invitrogen) along with 45 ng of Gα16 chimera containing the last 44 amino acids of rat gustducin (Gα16gust44) with 0.5 ul Lipofectamine 2000 (Invitrogen) per well. Cells are then incubated 22-44 hours at 37° C. 5% CO₂.

The expression of fTAS2R38 is evaluated by testing for the presence of a functional response to a known hTAS2R38 ligand (e.g., PTC), determined via automated calcium imaging using a Fluo-4AM (Life Technologies Corporation) Calcium Assay. Fluo-4AM is a fluorescent indicator of intracellular calcium dynamics (change in concentration) and allows monitoring changes in the calcium concentration, particularly an increase in response to receptor activation occurring after agonist exposure.

Generation of cell lines transiently expressing the other fTAS2Rs disclosed herein is analogous.

Expression of the fTAS2Rs in the various cell lines generated is evaluated by flow cytometry. The extracellular FLAG tag was detected with a FLAG-specific antibody conjugated to fluorescein isothiocyanate (FITC). The percentage of cells expressing a given fTAS2R is determined by percent of cells positive for the FITC signal. The level of fTAS2R expression is determined by the geometric mean of the fluorescence intensity measured. The results for each of the expressed fTAS2R are shown in Table 7.

TABLE 7 Flow cytometry results for cell lines transiently expressing fTAS2Rs % of cells Relative fTAS2R Expression expressing level (Geometric Mean of fTAS2R fTAS2R fluorescence intensity) Untransfected 0 8,929 cells TAS2R1 38 231,625 TAS2R2 37 295,625 TAS2R3 24 201,000 TAS2R4 36 331,125 TAS2R7 27 144,375 TAS2R9 24 113,250 TAS2R10 30 298,500 TAS2R12 32 258,625 TAS2R38 31 268,750 TAS2R42 25 133,375 TAS2R43 24 246,375 TAS2R44 9 125,750 TAS2R67 12 118,000

Testing for a functional response of fTAS2R38 to the known hTAS2R38 ligands, PTC and PROP, and of fTAS2R43 to the known hTAS2R43 ligands, aloin, denatonium and saccharine), was determined via automated calcium imaging using the Fluo-4AM (Life Technologies Corporation) Calcium Assay.

The fTAS2R38 was activated 81% over baseline by 100 μM PTC, but was not stimulated by 30 μM PROP. The fTAS2R43 was activated 45% over baseline by 300 μM aloin, and 17% over baseline by 1 mM denatonium, but was not stimulated by 6.7 mM saccharin. Furthermore, the responses to PTC, denatonium and aloin were inhibited by 1 mM probenecid.

Testing for a functional response of each of the other fTAS2Rs disclosed herein is performed by analogous methods using known ligands to a corresponding homolog of each fTAS2R.

C. Generation of Cell Lines Stably Expressing fTAS2R

Cell lines stably expressing fTAS2R are also obtained.

For these experiments, the fTAS2R38 cDNA is subcloned into an expression cassette based on the plasmid/expression vector pcDNA3.1Zeo (Life Technologies, Carlsbad, Calif., US), which contains within its multiple cloning sites the nucleotide sequence coding for the first 45 amino acids of the rat somatostatin receptor subtype 3 (RSS tag) to facilitate cell surface targeting of the transgene, and the nucleotide sequence coding for the herpes simplex virus (HSV) glycoprotein D epitope (HSV epitope) for facilitating immunocytochemical detection (HSV Tag).

The nucleic acid sequences encoding the RSS tag, TAS2R38, and the HSV tag are fused, in that order, in frame to create a construct to allow translation into the receptor protein. The resulting receptor cDNA in the expression vector encodes the joined amino acid sequences of TAS2R38 preceded by the RSS tag and followed by the HSV tag.

In a variation, the nucleic acid sequences encoding the RSS tag, HSV tag, and fTAS2R38 are fused, in that order, in frame to create a construct to allow translation into the receptor protein. The resulting receptor cDNA in the expression vector encodes the joined amino acid sequences of fTAS2R38 preceded by the RSS tag and the HSV tag.

The expression vector including the construct encoding the RSS tag, HSV tag, and fTAS2R38 in that order is called pcDNA3.1Zeo-TAS2R38 and allows for expression of the fTAS2R38 protein (SEQ ID:18).

Generation of an expression vector for the other fTAS2Rs disclosed herein is analogous. The restriction enzymes used are adapted accordingly.

Cell lines that stably express a desired fTAS2R disclosed herein are generated by transfecting the appropriate expression vector, e.g., pcDNA3.1Zeo-TAS2R38, constructed as described above into a eukaryotic host cell line (Life Technologies Cat #R700-07) transformed with the Gα16 chimera containing the last 44 amino acids of rat gustducin, Gα16-gustducin 44 cells, formed as described in WO2004/055048.

On day 0, the Gα16-gustducin 44 cells are plated in a 6-well plate at a density of 900,000 cells per well and grown overnight in a selective growth media (DMEM with 10% (v/v) heat-inactivated fetal bovine serum, 2 mM L-glutamine, 100 units/ml penicillin, 100 μg/ml streptomycin).

On day 1, the medium is exchanged with 2 ml of antibiotic-free and serum-free growth medium. 10 μl Lipofectamine 2000 (Life Technologies Corporation) is dissolved in 250 μl DMEM and incubated for 5 minutes at room temperature. In parallel, 4 μg pcDNA3.1Zeo-TAS2R38 DNA is dissolved in 250 μl DMEM. These two resulting solutions are mixed and incubated for 20 minutes at room temperature before they are added to the cells into the cell culture medium. After 4 hours, the medium is replaced with antibiotic-free, serum-containing growth medium. The cells are incubated in humidified atmosphere (37 C., 5% CO2).

After 24 hours, the cells are re-plated in selective growth medium (DMEM with 10% (v/v) heat-inactivated fetal bovine serum, 2 mM L-glutamine, 100 units/ml penicillin, 100 μg/ml streptomycin, 200 μg/ml G418 and 200 μg/ml zeocin) and are further incubated in a humidified atmosphere (37 C., 5% CO2).

After 2 to 4 weeks of culture (replacing medium as necessary), zeocin-resistant colonies are selected and expanded.

The expression of fTAS2R38 is evaluated by testing for the presence of a functional response to a known hTAS2R38 ligand (e.g., PTC and PROP), determined via automated calcium imaging using the Fluo-4AM (Life Technologies Corporation) Calcium Assay. Fluo-4AM is a fluorescent indicator of intracellular calcium dynamics (change in concentration) and allows monitoring changes in the calcium concentration, particularly an increase in response to receptor activation occurring after agonist exposure. One clone is selected resulting in the Gα16-gustducin 44/TAS2R38 cell line. The Gα16-gustducin 44/TAS2R38 cell line was activated 90% over baseline in the presence of 100 μM PTC but was not activated with 30 μm PROP.

Generation of cell lines stably expressing the other fTAS2Rs disclosed herein is analogous

Example 3 Cell-Based Screening for Ligands and Effectors of Feline TAS2Rs

Identification of agonists, antagonists and modulators of feline TAS2R38 receptor is performed by a cell-based screening assay in which the effect of a test compound on cells transfected with feline TAS2R38 and Gα16gust44 is compared against the effect of the test compound on untransfected cells.

Prior to the screening assay, the cells are loaded with the calcium sensitive dye Fluo-AM (Life Technologies) for one hour at 37° C. as described in Example 2B. The dye is washed out and the cells are assayed in Hank's Balanced Salt Solution (HBSS; Life Technologies) containing 20 mM HEPES in a Flexstation II (Molecular Devices). A 10 fold dilution series 0.01 mM-1 mM of test compounds is used to stimulate the cells. PTC, a known human TAS2R38 agonist, is among the test compounds

The stimuli are injected and monitored for 100-180 seconds. Data are analyzed and graphed as a percentage over the baseline signal, which is the reading prior to stimulation. Stimulation of the fTAS2R38 expressing cell line by a particular test compound is considered to occur when the signal is greater than both the signal from the buffer alone in the receptor expressing cell line and the signal from the un-transfected cell line sample injected with the test compound.

Cell based screening for agonists, antagonists, and modulators for the other fTAS2Rs disclosed herein is analogous.

The terms “a” and “an” do not denote a limitation of quantity, but rather denote the presence of at least one of the referenced item. The term “or” means “and/or”. The terms “comprising”, “having”, “including”, and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to”). The modifier “about” used in connection with a quantity is inclusive of the stated value and has the meaning dictated by the context (e.g., includes the degree of error associated with measurement of the particular quantity).

Recitation of ranges of values are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. The endpoints of all ranges are included within the range and independently combinable.

Unless defined otherwise, technical and scientific terms used herein have the same meaning as is commonly understood by one of skill in the art to which this invention belongs.

All references are incorporated by reference herein.

Embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of these embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. 

The invention claimed is:
 1. A polypeptide comprising the amino acid sequence of SEQ ID NO:2, or an amino acid sequence having at least 95% homology to the sequence of SEQ ID NO:2; wherein the polypeptide is covalently bound to a label, a solid support, or a lipid monolayer.
 2. The polypeptide of claim 1 comprising the amino acid sequence of SEQ ID NO:2.
 3. The polypeptide of claim 1 comprising an amino acid sequence having at least 95% homology to SEQ ID NO:2.
 4. The polypeptide of claim 1, wherein the polypeptide is covalently bound to a lipid monolayer.
 5. The polypeptide of claim 1, wherein the polypeptide is covalently bound to a label.
 6. The polypeptide of claim 5, wherein the label is a heterologous polypeptide sequence.
 7. The polypeptide of claim 6, wherein the heterologous polypeptide sequence is a membrane targeting sequence or a peptide tag sequence.
 8. The polypeptide of claim 5, wherein the label is a radiolabel, a fluorescence label, a chemiluminescent label, an enzymic label, or an immunogenic label.
 9. The polypeptide of claim 1, wherein the polypeptide is covalently bound to a solid support.
 10. A method for identifying a compound that interacts with a feline TAS2R receptor comprising: contacting the polypeptide of claim 1 with a test compound, and detecting interaction between the polypeptide and the test compound.
 11. The method of claim 10, wherein the polypeptide is bound to a solid support or expressed in a host cell.
 12. A method for identifying a compound which modulates a feline TAS2R receptor which comprises: contacting a polypeptide of claim 1 with a receptor ligand in the presence or absence of a test compound, and determining whether the test compound modulates binding of the ligand to the polypeptide or activation of the polypeptide by the ligand.
 13. A chimeric polypeptide comprising at least one extracellular domain of SEQ ID NO:2 covalently linked to a heterologous polypeptide sequence, wherein the extracellular domain comprises amino acids 68-84; 146-179; or 249-257 of SEQ ID NO:2.
 14. A chimeric polypeptide comprising at least one intracellular domain of SEQ ID NO:2 covalently linked to a heterologous polypeptide sequence, wherein the intracellular domain comprises 23-46; 106-124; 201-227; or 279-298 of SEQ ID NO:2.
 15. A chimeric polypeptide comprising at least one transmembrane domain of SEQ ID NO:2 covalently linked to a heterologous polypeptide sequence, wherein the transmembrane domain comprises amino acids 2-22, 47-67, 85-105, 125-145, 180-200, 228-248, or 258-278 of SEQ ID NO:2.
 16. The chimeric polypeptide of claim 15 comprising the transmembrane domains comprising amino acids 85-105 of SEQ ID NO:2, amino acids 228-248 of SEQ ID NO:2, and amino acids 258-278 of SEQ ID NO:2.
 17. The chimeric polypeptide of claim 15 comprising the transmembrane domains comprising amino acids 85-105 of SEQ ID NO:2, amino acids 180-200 of SEQ ID NO: 2, amino acids 228-248 of SEQ ID NO:2, and amino acids 258-278 of SEQ ID NO:2.
 18. The chimeric polypeptide of claim 15 comprising the transmembrane domains comprising amino acids 85-105 of SEQ ID NO:2, amino acids 125-145 of SEQ ID NO:2, amino acids 180-200 of SEQ ID NO: 2, amino acids 228-248 of SEQ ID NO:2, and amino acids 258-278 of SEQ ID NO:2.
 19. The chimeric polypeptide of claim 15 comprising the transmembrane domains comprising amino acids 2-22 of SEQ ID NO:2, amino acids 47-67 of SEQ ID NO:2, amino acids 85-105 of SEQ ID NO:2, amino acids 125-145 of SEQ ID NO:2, amino acids 180-200 of SEQ ID NO: 2, amino acids 228-248 of SEQ ID NO:2, and amino acids 258-278 of SEQ ID NO:2.
 20. The chimeric polypeptide of claim 19 further comprising amino acids 68-84 of SEQ ID NO:2. 